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Can Yafo Lift Speed Limits?

Light Reading
News Analysis
Light Reading
2/19/2001
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Much has been made of the need for equipment vendors to offer OC192 (10 Gbit/s) interfaces and to be first out of the gate with the next step up in transmission speed, OC768 (40 Gbit/s).

Very little has been said about polarization mode dispersion (PMD), a problem that could prevent a lot of carriers from upgrading their networks with these new technologies.

Now, however, startup Yafo Networks reckons it’s close to offering carriers a way around the PMD problem -- a compensator that will enable them to deploy higher-speed gear on long-distance routes.

Early prototypes of the PMDC-10, as it’s called, are already being evaluated by “four or five” vendors thinking of incorporating it in their equipment, according to Henry H. Yaffe, Yafo Networks’ founder, chairman, and CTO. Commercial prototypes will ship in “a couple of months.”

Developing a PMD compensator is a big deal, mainly because PMD is much tougher to control than other forms of dispersion, due to which signals become increasingly smudged and unreadable as they travel along fiber. This is caused by some parts of the light pulse traveling slightly faster than other parts.

As the name implies, PMD is caused by light traveling faster in one polarization plane than another. Fundamentally, it’s caused by the core of the fiber not being perfectly round in cross-section. As a result, the thickness isn’t absolutely identical on every possible axis.

Up until about five years ago, manufacturers simply couldn’t produce fiber that was circular enough to keep PMD under control at bandwidths exceeding OC48 (2.5 Gbit/s), according to Yaffe. Fiber happens to have a “sweet spot” at OC48 that prevents PMD being an issue, he says. At higher speeds, however it becomes increasingly problematic. It’s “pretty bad” at OC192 and creates a “real mess” of signals at OC768.

The upshot is that carriers with older fiber in their backbones –- like AT&T Corp. (NYSE: T), Sprint Corp. (NYSE: FON), and WorldCom Inc. (Nasdaq: WCOM) -- are now lumbered with infrastructure that can’t really support the latest transmission technologies, Yaffe says.

Other carriers also face problems because otherwise circular cross-section fiber can be squashed out of shape during installation or at any time afterwards, either permanently or temporarily. A lot of fiber is laid alongside railway tracks, for instance, and vibrations from passing trains can flatten it slightly, creating temporary PMD problems, according to Yaffe.

The fact that PMD problems can come and go like this makes it tough to compensate for them. Other dispersion problems don’t change over time and can be dealt with by installing static compensators. PMD compensators, however, have to work dynamically. Yafo's PMDC-10 continuously adjusts the orientation of the light signals and the time difference between the two polarization planes to optimize the signal quality, says Yaffe.

The same concept could be used for automatically adjusting other transmission parameters when traffic is rerouted around a problem in an optical backbone, Yaffe notes. "This is a platform of technologies that lends itself to a whole plethora of applications,” says Yaffe.

Downsides? The PMDC-10 occupies a lot of real estate. Each wavelength requires a whole blade of components, and there are 10 blades per shelf and three shelves per bay. In other words, PMD compensation for a 160-channel DWDM system might occupy as many as six bays of equipment. Yafo Networks isn’t talking about price yet, but it’s a fair bet that it won’t come cheap.

At least one other startup, Phaethon Communications, is thought to be developing a PMD compensator. Phaethon is still in stealth mode, but its Website says that it’s “devoted to delivering OC192 and OC768 data rates over all types of fiber.”

-- Peter Heywood, international editor, Light Reading http://www.lightreading.com

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Peter Heywood
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Peter Heywood,
User Rank: Light Beer
12/4/2012 | 8:52:21 PM
re: Can Yafo Lift Speed Limits?
This story identifies 2 vendors making PMD compensator subsystems - Yafo and Phaethon. Let me know of any others.

Beneath these guys, there's another tier of vendors making the components that do all the clever stuff, like rotating the polarization axis and shifting the time difference between the different polarization planes. One such outfit in this field is General Photonics. Let me know of any others.

Question: Are system vendors going to buy ready-made subsystems from the likes of Yafo and Phaethon, or are they going to build PMD compensation into their own equipment using components from the likes of General Photonics? Who's taking what approach?

[email protected]
pingu
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pingu,
User Rank: Light Beer
12/4/2012 | 8:52:21 PM
re: Can Yafo Lift Speed Limits?
Hi Peter,

Nice article on Yafo.

I also came across a article in Chase H&Q's "Connected" publication called "OC-768: The Next Speed Bump" which discussing many of the technical challenges of 40 Gbs systems. LightReading, of course, has addressed this issue as well, for instance Scott Clavenna's "Top Ten Trends" piece.

Here is the link to the Connected article:

http://connected.jpmhq.com/iss...

I also found a good link from the Yafo Network site discussing some of the technical aspects of PMD:

http://www.yafonet.com/pages/a...

However, I haven't been able to find any tech sheets or detailed product information on Yafo's PMD compensators. The PR's discuss some of its aspects:

"The Yafo10 PMD compensator uses adaptive optical networking technology to alter the character of the bit-stream of light in real-time and overcome the PMD problem."

"As data rates of fiber optic networks increase to 10 Gbps and higher, the optical signal becomes distorted by polarization mode dispersion particularly as it passes through long haul legacy fiber networks. The EyeOpener alleviates this problem by adaptively canceling the polarization mode dispersion at the receiver. This results in a cost effective and highly reliable solution that greatly improves the transmission quality for service providers and network equipment manufacturers.

[GǪ]

Additional features of the EyeOpener include a small footprint, low power dissipation and a solid-state design."

But I can't find any info as to this product's technical performance. Do you know

The Avanex's PowerShaper family of products primarily addresses the problem of chromatic dispersion, but also makes the claim of "low PMD and low ripple". The link to the tech information is below:

http://www.avanex.com/products...

General description of Avanex's PowerShaper:
-----------------------------
PowerShaperGäó Dispersion Management Processors

"The PowerShaper broadband, fixed and variable chromatic dispersion compensation and dispersion slope compensation processors are designed to correct bandwidth and distance limitations inherent in the transmission of optical signals in conventional optical fibers. Dispersion is variations in the speeds of different wavelengths of light as they travel over distance. Dispersion generally limits optical signal transmission to 100 kilometers without signal regeneration. Conventional signal regeneration requires the light be converted to electrical pulses for the signal to be boosted and then reconverted to light before being forwarded--a significant roadblock on the Information Highway. The PowerShaper is designed to restore the integrity of the light signals, allowing signals to be transmitted significant distances without electrical regeneration. With DWDM providing more and more channels within a single fiber, the PowerShaper prevents signals from mixing and corrupting transmission data over the long haul."
---------------------

My questions are:

Isn't chromatic dispersion a relatively greater problem than PMD?
Would the low PMD characteristics of Avanex's PowerShaper (see tech sheet) obviate the need for Yafo's PMD compensator?
Will networks wanting to deploy 40Gbs systems require separate components (filling yet more racks) to tackle these two problems (chromatic and polarization mode dispersion), or is an integrated approach possible?

Thanks and Regards,
pingu
Petabit
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Petabit,
User Rank: Light Beer
12/4/2012 | 8:52:19 PM
re: Can Yafo Lift Speed Limits?
pingu:"Isn't chromatic dispersion a relatively greater problem than PMD?
Would the low PMD characteristics of Avanex's PowerShaper (see tech sheet) obviate the need for Yafo's PMD compensator?
Will networks wanting to deploy 40Gbs systems require separate components (filling yet more racks) to tackle these two problems (chromatic and polarization mode dispersion), or is an integrated approach possible?"

There are a lot of good questions there.

Chromatic dispersion is a much bigger problem than PMD. Typical fibre chromatic dispersion is about 15 ps/nm/km. So for a 40nm wide system, and a 80 km span, you get somewhere around 50,000 ps of dispersion. Typical PMD is about 1 ps for an 80km span.

Chromatic dispersion in in theory really easy to fix, you just find an element with the opposite dispersion. Dispersion compensating fibre has been around for ages, and works well. The cost comes from the manufacturing control required to make the DCF accurately and repeatably. So to fix chromatic dispersion (both magnitude and slope) you just need to throw money at it - the technology exists.

Interestingly the chromatic dispersion compensation is much better for plain old NDSF (SMF-28) than any of the newer fibres like TrueWave or LEAF. In really high capacity systems, you might need an adjustable dispersion compensator to hit the value accurately enough to get the system to work. You will probably need one adjustable DCM for each group (maybe each channel) in the system.

PMD is a different matter. It changes with time. It comes from the fibre itself, but also from how you lay and store the fibre. It comes from every component (especially isolators and circulators) and can change at varying rates.

If you have decent fibre (post 1992) and it is laid well, then PMD is not a problem even at 40G. However if you want to use old or dodgy fibre, you will need a PMD compensator. The compensator needs to be fast (10-50 Hz response time) and yo have to have one for each channel.

Today you could use both a chromatic and PMD compensation device, they could both be active, and would be required for every channel. The cost of these compensators along would double the entire cost of the system. Which is why no-one uses them.

Yafo have gone public with a devie that has been build as one-offs by most of the systems vendors (take a look at the papers at OFC 1998). There are plenty of 10G systems out there that are working fine without PMD compensators. And the 40G trials don't use them either. Yafo will have the make the device really cheap to get people to buy it in any volume.

P.

PS: control. All the PMD compensators that I've seen require a feedback signal from the receiver to drive them. Are Yafo going to build a new interface for each an every OEM?
Oakster
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Oakster,
User Rank: Light Beer
12/4/2012 | 8:52:14 PM
re: Can Yafo Lift Speed Limits?
Petabit: You write...

"Yafo will have the make the device really cheap to get people to buy it in any volume."

How cheap?

Oakster
flip wilson
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flip wilson,
User Rank: Light Beer
12/4/2012 | 8:52:12 PM
re: Can Yafo Lift Speed Limits?
P

optisphere is introducing OC 768 with PMC compensation.

A white paper on compensating for PMC by using special fiber is here:

http://www.furukawa.co.jp/revi...


FW
abarbier
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abarbier,
User Rank: Light Beer
12/4/2012 | 8:52:11 PM
re: Can Yafo Lift Speed Limits?
"Isn't chromatic dispersion a relatively greater problem than PMD?"

at 40Gbps PMD is relatively grater.

"Will networks wanting to deploy 40Gbs systems require separate components (filling yet more racks) to tackle these two problems (chromatic and polarization mode dispersion), or is an integrated approach possible?"

Not necessarily: for CD you could use a "simple"
Fiber Bragg Grating and for PMD there is something
(among others) called Tunable Non-Linearly High-Birefringent Fiber Bragg Grating which won't
eat rack space.
abarbier
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abarbier,
User Rank: Light Beer
12/4/2012 | 8:52:11 PM
re: Can Yafo Lift Speed Limits?
Hi gang,

"Typical PMD is about 1 ps for an 80km span."

on a good fiber PMD is <0.5 ps/square_root(Km)
on many installed fibers PMD is >10 ps/square_root(Km)

The real problem with PMD is that it is not predictable unlike CD. A guy at bell-labs showed
how a mere 1 celsius of temperature variation can
alter PMD. Even a train rolling over the rail where the fiber lies can change the conditions.

And the bad thing is that at 10Gbps and 40Gbps it is a damn serious issue.

IMHO the fact that in trials people have demonstrated 40Gbps, means absolutely zero!!
The real world is made of 95% old SMF fiber and the variance of PMD is scary. You need DYNAMIC compensation of PMD.

"There are plenty of 10G systems out there that are working fine without PMD compensators. And the 40G trials don't use them either. Yafo will have the make the device really cheap to get people to buy it in any volume."

I bet that at 40Gbps there will be no real commercial system without PMD dynamic compensation.

cheers
bockwai
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bockwai,
User Rank: Light Beer
12/4/2012 | 8:52:10 PM
re: Can Yafo Lift Speed Limits?
For those interested, there was a very informative article from the founders of General Photonics in a Fiberoptic Product News archive from October 2000 called Overcoming Polarization Impairments. Its a highly analytical piece on PMD compensation. Here's the link to the article:

http://www.fpnmag.com/exec/rel...

bockwai
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bockwai,
User Rank: Light Beer
12/4/2012 | 8:52:10 PM
re: Can Yafo Lift Speed Limits?
Sorry, bad link. Lets try it again without using a link:
Overcoming Polarization Impairments
By Chao Yeh, Gang Yang, and Steve Yao, General Photonics Corporation

Polarization-related impairments have become the major obstacles to the increase of transmission rates in WDM systems. Such impairments include polarization mode dispersion (PMD) in optical fibers, polarization dependent loss (PDL) in passive optical components, polarization dependent modulation (PDM) in electro-optic modulators, and polarization dependent gain (PDG) in optical amplifiers. A dynamic polarization controller is identified as the single most important element in various schemes for overcoming these impairments.

The cause for these polarization impairments is the imperfection of the optical fibers. If the fibers were perfect, the state of polarization (SOP) of the light signal transmitting in the fiber would remain constant and the effects of PMD, PDL, PDM, and PDG could easily be eliminated. Unfortunately, the SOP of light propagating in a length of standard communication fiber varies along the fiber due to the random birefringence induced by the thermal stress, mechanical stress, and irregularities of the fiber core. Generally, at the output end of the fiber the light is elliptically polarized, with varying degrees of ellipticity, and with the major elliptical axis at an arbitrary angle relative to some reference orientation. Worst of all, the induced birefringence changes with temperature, pressure, stress and other environmental variations, making polarization related impairments time dependent.


Figure 1. The polarization stabilizer has an input port, an output port, and two device ports for accepting the two ends of a polarization sensitive device, such as a LiNbO3 modulator. The stabilizer automatically adjusts the SOP to the modulator and ensures a maximum light transmission. The heart of the stabilizer is a fiber squeezer polarization controller.


Figure 2. Fiber squeezer dynamic polarization controllers have the advantages of no insertion loss, no backreflection, no PDL, extremely low activation loss, and high speed.

Polarization Mode Dispersion (PMD)
PMD is often cited as the next critical hurdle for the high bit rate transmission systems (10 Gb/s and higher) after chromatic dispersion and fiber nonlinearity impairments are successfully managed. As illustrated in Figure 3, a fiber link can be considered as a concatenation of many, randomly oriented retardation plates. In the absence of PDL or PDG in the fiber link, these retardation plates are optically equivalent to a single retardation plate with an effective DGD and a pair of effective orthogonal principal axes (which can either be linear or circular) for a given optical frequency. Upon entering the retardation plate, an optical pulse is decomposed into two polarization components along the two axes. Because the two components travel with different speed in the retardation plate, they exit the plate with a relative time delay called Differential Group Delay (DGD). When DGD is comparable with the bit separation of a data stream, bit error rate may significantly increase.


Figure 3. A real fiber is like many retardation plates in series with different orientations and birefringence. It is equivalent to a single retardation plate having a slow and fast axis and an equivalent birefringence. An optical pulse broadens because the two polarization components travel at different speeds.

The rms value of the DGD is often referred as first order PMD. Similar to the famous random walk problem in which the rms distance of a drunken man to the origin is proportional to the square-root of the number of walking steps, PMD is proportional to the square-root of the number of cascaded retardation plates - or equivalently - to the square-root of the fiber length.

Contrary to the case of a true retardation plate, the DGD and the principal axes of the fiber link depend on the wavelength and fluctuate in time as a result of temperature variations and external constrains. Consequently, the corresponding pulse broadening is random, both as a function of wavelength at a given time and as a function of time at a given wavelength. As a rule of thumb, the maximum tolerable DGD value is 14% of the bit duration to ensure an outage probability of less than 5 minutes per year at a 3-dB power penalty. This translates to 14 ps for a 10 Gb/s system and 3.5 ps for a 40 Gb/s system. Unfortunately, for link distance greater than 300 km, 20% installed fiber plant is not suitable for 10 Gb/s transmission and 75% is not suitable for 40 Gb/s for such an outage. Therefore, PMD compensation is required for these fiber links.

Unlike the effects of chromatic dispersion and fiber nonlinearity, which are deterministic and stable in time, the PMD-induced penalty can be totally absent at any given moment and adversely large several days later to cause unacceptable bit-error-rate for no apparent reasons. To ensure an acceptable outage probability for the fiberoptic system, PMD compensation must be dynamic in nature and adaptive to the random time variations. Various schemes for mitigating the effect of PMD are illustrated in Figure 4. The schemes generally contain three key components: 1) the dynamic polarization controller (DPC), 2) the PMD analyzer, and 3) the feedback circuit. For some schemes, the dynamic variable delay line may also be required. The low insertion loss specification of the fiber squeezer-based device becomes even more attractive for the schemes that require multiple polarization controllers capable of compensating higher order PMD effects. As will be discussed below, low PDL and low activation loss are also important parameters for the DPC to function properly in the feedback loops.


Figure 4. Various PMD mitigation schemes: a) Principal state transmission method; b) Multiple retardation plates method; c) Optical delay line method; d) Electrical delay line method; e) Chirped HiBi fiber grating method. In all schemes, dynamic polarization controllers (DPC) having fast speed, low PDL, and low activation loss are required.

Polarization Dependent Loss (PDL)
The PDL of an optical component is defined as the difference between the maximum and the minimum insertion losses for all possible input SOPs. Optical components with PDL act as partial polarizers with two orthogonal axes (either linear or circular). A light signal experiences a maximum loss if its SOP is aligned with one of the axis and a minimum loss if aligned with the other axis.

Almost all fiberoptic components have PDL and the causes may be different for different components. First of all, when light from an optical medium with an index of n1 is transmitted to another medium with an index n2, reflection occurs. The reflection coefficients for the polarization states particular and parallel to the plane of incidence are different if the angle of incidence is not normal. (Many fiber components have angled input and output surfaces for increased return loss.) Such a difference in reflection results in a difference in transmission loss or PDL. For example, an 8-¦ angle polished connector (FC/AC or SC/APC) has a PDL of 0.022 dB. Fiber grating-based devices may also exhibit PDL if the grating is not normal to the fiber longitudinal axis.

Second, for many optical components, such as isolators and circulators, birefringent crystals are often used. Because a birefringent crystal has two principal axes with different indexes of refraction, no and ne, the Fresnel reflection coefficients of two polarization states perpendicular and parallel to the principal axes are different even at normal incidence, resulting in different transmission loss. The corresponding PDL is thus:
PDL=20*log|(ne-1)(no+1)/(no-1)(ne+1)
Anti-reflection (AR) coating can greatly reduce the reflection; however, it may not be totally eliminated PDL because the optimal coating layer thickness is determined by the refractive index of the coated material: it is either optimized for no or ne.

Diffraction grating-based optical components or instruments generally have high PDL because the diffraction efficiencies for the two polarization states perpendicular and parallel to the plane of incidence are different.

Finally, any fiber component containing a dichorism material also has PDL. A dichorism material has two principal axes with different absorption or attenuation coefficients. The principal axes can either be linear (linear dichorism) or circular (circular dichorism). For example, the LiNbO3 waveguide made with the proton exchange method exhibits strong linear dichorism and acts just like a polarizer.

In a fiber link which contains many optical components with different PDL values, the total PDL value depends on the SOPs of the light signal transmitted in the link and varies between a maximum and a minimum value. The maximum value equals to the summation of the PDL values of all the components in the link. It is the difference between two insertion loss measurements of the link: the first one is when SOP of the light before each component is aligned with the minimum loss axis of the component and the second one is when SOP of the light before each component is aligned with the maximum loss axis.

The minimum PDL value corresponds to the case that the SOPs of a light signal before all the PDL components are such arranged that the PDLs cancel one another out. The net residual PDL is the minimum PDL of the link.

The presence of PDL in a fiber link also complicates PMD compensation. When the PDL is present in the fiber link, the link is no longer equivalent to a single retardation plate. It instead is equivalent to two retardation plates with a partial polarizer sandwiched in between. Any PMD compensation scheme therefore must take the effect of the partial polarizer into account, which may increase the complicity of the compensation arrangement significantly.

Similar to PMD penalty, the PDL effect in a fiber link containing multiple PDL components separated by sections of singlemode fiber is also time dependent. At any given moment, the states of polarization in different sections of the link may be oriented favorably to allow a low link loss such that the detected optical power at the receiver is high enough to achieve acceptable bit-error rate (or signal-to-noise ratio). However, at a different moment, the link loss may be too high to achieve quality transmission due to the external thermal or mechanical stress on the fiber that causes the states of polarization in different fiber sections to re-arrange.

To combat the time-varying PDL problem, dynamic polarization controllers may be used at selected locations in the fiber link, as shown in Figure 5. Each polarization controller assures that the light passing through the PDL component following the controller has the lowest loss.


Figure 5. Dynamic Polarization Controllers (DPC) for a) Polarization stabilization; b) PDL compensation; and c) PDG mitigation.

Polarization Dependent Gain (PDG)
The gain of an optical amplifier for the stronger polarization component is less than that for the weaker component (because the stronger component saturates the gain more) and the gain difference is called polarization dependent gain (PDG). One cause for the PDG is that the cross sections of the simulated emission for different polarization states are different. This polarization hole burning always gives more gain to the weaker polarization component and thus tends to cause the polarization state to change with time. In addition, when the input SOP changes, the signal gain may increase temporarily and come down in a short period of time. Consequently, the polarization hole burning always encourages polarization fluctuations in a fiber laser system and thus causes mode-hopping and increases super-mode noise in a mode-locked laser.

An optical amplifier may, at the same time, exhibit PDL effect. For example, couplers and isolators are generally contained in an Er+ doped fiber amplifier (EDFA) and the presence of PDL in these components gives rise to the apparent PDL of the amplifier. Even in semiconductor optical amplifiers (SOA), the facets of the semiconductor chip are generally angle cleaved to prevent optical feedback into the amplifier. As discussed previously, these angled interfaces exhibit large PDL, which directly contributes to PDL of the SOA.

In a fiberoptic link with many optical amplifiers and many components with PDL, the effect of PDG can be significant at some moments and negligible at other times. When a large number of optical amplifiers are cascaded in a long haul fiber link, the performance degradation caused by PDG is significant even though each amplifier may have a very small PDG (~0.1 dB). The performance degradation becomes even worse when PDG is combined with the PMD and PDL of the fiber and other components in the link. Polarization scrambling at a frequency above the amplifier's response rate (inverse of amplifier's upper energy level life time, ~ 500 Hz for EDFA) has proven to be effective in mitigating PDG impairment in long haul systems, as shown in Figure 5. A factor of 2 increase in systems Q factor was demonstrated in an 8100-km link containing 181 EDFAs with such a scheme.

Polarization Dependent Modulation (PDM)
In addition to PDL, external modulators, such as LiNbO3 based electro-optical modulators and semiconductor electro-absorption modulators, also exhibit polarization dependent modulation in that the modulation depth of the signal with different polarization states are different. As a result, the amplitude of the received data bits varies when the state of polarization of light before entering the modulator varies due to the fluctuation of temperature or other external constrains on the fiber, resulting in bit-error-rate fluctuation.

To assist polarization alignment, most Ti-indiffused LiNbO3 modulators embed a polarizer at input or out of the waveguide and thus convert the PDM problem into a more easily identified PDL problem. The LiNbO3 modulators made with proton exchange process act like a polarizer itself without the embedded polarizer. Thus one method of eliminating PDM effects uses a fast response dynamic polarization controller placed in front of the modulator to assure that light passing through the modulator has minimal loss, as illustrated in Figure 5.

"Dynamic" Polarization Control
Because polarization-induced penalties are time dependent, polarization-impairment mitigation must be dynamic and adaptive to random time variations. A dynamic polarization controller is the single most important element for overcoming these impairments.

Fast speed, low PDL, low insertion loss, and low activation loss are all critical parameters in evaluating a dynamic polarization controller. Activation loss measures the additional insertion loss caused by activation of the device and is defined as the difference of the maximum and minimum insertion losses of the device considering all possible activation conditions. This specification is particularly important because all polarization-impairment compensation schemes utilize a feedback signal to activate the polarization controller. The activation-induced loss causes errors in the feedback signal and directly degrades the performance of the compensation apparatus. In addition, when an instrument for measuring the PDL of optical components includes a polarization controller, the activation-induced loss limits the resolution and accuracy of the measurement. Similarly, polarization controllers' PDL also contributes to errors in a feedback system and complicates the design of compensation hardware and software.

Present dynamic polarization controllers on the market include free-space retardation plate-based and Lithium Niobate waveguide-based devices. The free-space devices contain multiple (sometimes three) retardation plates with different relative orientations. Applying a voltage to each retardation plate changes its retardation and hence the polarization of light passing through the plate. Because light has to exit the fiber, collimated, passed through the plates and finally focused back into the fiber when making such devices, the resulting labor costs, material costs, and insertion losses are high. The retardation plates may be made of liquid crystal or solid state electro-optical materials. Liquid crystal devices also suffer from low speed (10 to 100 ms), narrow operating temperature ranges, and high PDL.

Lithium Niobate-based dynamic polarization controllers contain multiple sections of waveguides with different electrode (or crystal) orientations. Similar to the liquid crystal based devices, each waveguide functions as an electrically variable retardation plate. By applying different voltages to different waveguide sections, any polarization states can be generated. However, typical of lithium niobate waveguides, such a controller has high insertion loss (~4 dB), high polarization dependent loss (~0.2 dB), low return loss (45 dB), high cost, and narrow temperature range (0 to 60-¦n addition, the maximum power of the device is limited to 50 mW, making it impractical to use after optical amplifiers.

Fiber Squeezer Polarization Controllers
Polarization controllers based on fiber squeezers have been investigated by researchers for over a decade. However, reliability concerns associated with fiber squeezing and high activation loss prevented such device from becoming commercially feasible. In 1996, General Photonics introduced a commercial fiber-squeezer based polarization controller (PolaRITE). Over the years, the company has developed several proprietary technologies dealing with fiber squeezing.

Based on prior experiences with fiber squeezers, General Photonics recently introduced a multi-axis fiber-squeezer based dynamic polarization controller/scrambler (shown in Figure 2). Similar to the bulk-wave plate-based and Lithium Niobate-based polarization controllers, the fiber-squeezer controller contains multiple retardation plates with different orientation angles. Each retardation plate is created by squeezing a section of fiber with a PZT actuator and the retardation of the plate can be easily changed by varying the voltage applied to the PZT actuator. It can be shown that this device can convert any polarization state to any desired polarization state on the Poincare Sphere by controlling the voltages on different fiber sections.

Due to its all fiber nature, this device practically has no insertion loss, no back reflection, and no polarization dependent loss. Such high speed operation is essential for tracking fast polarization variations which are caused either by the passing automotive as in fibers laid along railway tracks or by ocean waves as in the trans-oceanic fiber trunks.

Low activation-induced loss of 0.003 dBmakes these components ideal for use in high precision PDL instruments and in feedback loops for compensating for polarization induced penalties.

The device's performance is wavelength independent: the device functions equally well for signals ranging from 1280 nm to 1650 nm. This one-device-fits-all feature helps to simplify system design, lower implementation cost, and enables systems' channel expandability.

Implementation of fiber squeezers is also cost effective. Half-wave voltage requirements of the fiber squeezer controllers have been reduced to less than 40 VDC. The low voltage requirements allow the use of readily available low cost electronics to drive and control the fiber squeezer controller. Reliability of the fiber squeezers has also proven to be very high. In testing, over 10 billion activation cycles at half wave voltages have been achieved without a single failure.

An additional application of the fiber squeezer controller is as a polarization scrambler to effectively randomize polarization states. With a built-in resonant enhanced circuit, the half wave voltages of the device at scrambling frequencies are reduced to only a few volts. Such a low voltage requirement makes the driving electronics simple and low cost. With properly selected driving parameters, the scrambler has successfully achieved a polarization sensitivity of less than 0.05 dB and a degree of polarization less than 1%.


Applications
Due to their superior performance (low insertion loss, low PDL, low activation loss, low backreflection, high speed, and low cost), fiber squeezer dynamic polarization controllers are ideal for PMD mitigation, PDL compensation, and PDG effect reduction, as illustrated in Figures 4 and 5.

Fiber squeezer dynamic controllers can also be used for polarization stabilization, which is important for electro-optic and electro-absorption modulators, optical interferometers, and heterodyne optical receivers. The polarization stabilizer (Figure 1) uses the fiber squeezer-dynamic controller and has achieved output power stability to within 0.05 dB against all possible polarization variations.

The polarization scrambling feature of the fiber squeezer-based device can also be used to eliminate the instrument's polarization sensitivity. Some optical instruments, such as diffraction grating-based optical spectrum analyzers, are sensitive to the state of polarization of the input light. Scrambling the input polarization is an effective way of removing the measurement uncertainties caused by the polarization sensitivity. The fiber-squeezer based dynamic polarization controller/scrambler is ideal for such an application because of its low scrambling voltages and multiple resonant scrambling peaks. The scrambler has successfully achieved a polarization sensitivity of less than 0.05 dB and a degree of polarization less than 1%.

Fast and accurate PDL characterization of fiberoptic devices in a manufacturing environment is important. The dynamic polarization controller can be readily included in an instrument to automatically search for the maximum and minimum insertion loss a device under test and calculate the corresponding PDL. Fiber-squeezer based dynamic controllers are especially attractive for this application because of their low activation loss and low PDL, which ultimately limit the PDL measurement accuracy. Our test indicates that a PDL measurement with a resolution of 0.01 dB can be readily achieved in a fractional second using a dynamic controller.
Suggested Readings:

M. Chbat, "Managing polarization mode dispersion," Photonics Spectra, June 2000, pp. 100-104.
T. Ono and Y. Yano, "10 Gb/s PMD compensation field experiment over 452 km using principal state transmission method," Optics & Photonics News, p. 61, May 2000.
H. Bulow, "PMD mitigation techniques and their effectiveness in installed fiber," OFC'200 Proceedings, ThH1-1, p.110.
D.A. Watley et al, "Field evaluation of an optical PMD compensator using an installed 10 Gb/s system," OFC'2000 proceedings, ThB6-1, p.37.
Z. Pan et al, "Chirp-free tunable PMD compensation using Hi-Bi nonlinearly chirped FBGs in a dual-pass configuration," OFC'2000 Proceedings, ThH2-1, p113.
F. Bruyere, O. Audouin, V. Letellier, G. Bassier, and P. Marmier, "Demonstration of an optimal polarization scrambler for long-haul optical amplifier systems," IEEE Photonic Technoloy Letters, Vol. 6, pp. 1153-1155 (1994).
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Chao Yeh is a senior photonics engineer, and Gang Yang is an electronics design engineer with General Photonics Corporation. Steve Yao is CTO of the company.
pingu
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pingu,
User Rank: Light Beer
12/4/2012 | 8:52:08 PM
re: Can Yafo Lift Speed Limits?
Thanks Petabit!

Regards,
pingu
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