Volume Phase Holographic (VPH) Gratings for Telecommunications Applications
The purpose of this white paper is to discuss Volume Phase Holographic (VPH) transmission grating solutions specific to optical telecommunications applications. For more basic answers to questions such as:
- What is a VPH transmission grating?
- How is a VPH grating made?
- Why consider using a VPH grating?
- Why use Kaiser’s VPH gratings?…and
- What kinds of systems employ Kaiser’s VPH gratings?
…please see the note “Introduction to Volume Phase Holographic (VPH) Transmission Gratings and Applications”.
We briefly reiterate here just a few basic points. VPH transmission gratings work much like conventional surface relief reflection gratings, except in transmission, as shown in Figure 1. They are optically thick (hence the term volume) periodic phase structures, whose fundamental purpose is to diffract different wavelengths of light from a common input path into different angular output paths.
Figure 1. Conventional reflection vs. VPH transmission gratings.
Key points worth considering with regard to VPH vs. conventional gratings include:
- VPH gratings can provide an unprecedented combination of high dispersion, high efficiency, and low polarization sensitivity over the full telecom wavelength range.
- The transmission geometry enables compact, efficient system configurations.
- VPH gratings are rugged, cleanable, and may be qualified to extreme environments.
- Every VPH grating is a genuine holographic master, not a mechanical replication, resulting in high optical quality and low scatter.
- Flexible manufacturing infrastructure allows custom grating development and prototyping without the expense of mechanical replication tooling.
Any applications that require high-resolution separation of narrowly spaced DWDM wavelengths merit consideration of VPH gratings. These include:
- Reconfigurable Add/Drop Multiplexers (ROADMs)
- Dynamic Gain Equalizers
- Optical spectrum Analyzers
- Optical Channel Monitors
- Fiber Multiplexers/Demultiplexers (MUX/DEMUX)
- Diode laser wavelength tuning
There are many available technologies that can provide wavelength separation. Besides VPH transmission gratings, there are surface relief gratings, arrayed waveguide gratings (AWGs), fiber Bragg gratings (FBGs), and thin film filters. Relative advantages and disadvantages of each are summarized qualitatively in Figure 2.
Applications involving just a few channels, or widely spaced CWDM bands, may be most economically served by lower cost elements such as thin film filters.
Conventional reflective surface relief gratings can also be relatively inexpensive, and may be more cost-effective than thin film filters in many applications with larger channel counts. However, they are very easily damaged and limited in available efficiency/PDL/bandwidth performance. Input and output lie on the same side of a reflection grating, which can be fine in some applications, but extremely limiting in laying out some classes of devices, for example ROADMs.
To address this packaging/design limitation, some reflection grating manufacturers have recently begun to offer surface relief transmission gratings. This approach to transmission gratings presents several challenges of its own. Most significant is that the physical depth of a surface relief transmission grating must be on the order of four times that of a reflection grating in order to produce high efficiency. The resulting pattern is even more fragile than a reflection grating and tends to scatter even more light. It also sacrifices efficiency/PDL performance relative to reflection gratings due to, among other factors, the “shadowing” of the diffracted orders by the deep relief pattern.
Fiber Bragg gratings (FBGs) are very effective at selectively reflecting a single narrow channel of light. As such, they can be a very effective “plug and play” approach to stabilizing a laser diode wavelength, or adding/dropping a single fixed channel. However, configuring an add/drop of several channels with FBG technology typically requires a daisy chain of multiple FBGs and circulators. This can quickly become expensive, and performance tends to roll off for channels at the far end of the chain.
Arrayed waveguide gratings (AWGs) are in some ways an elegant planar lightwave circuit (PLC) analog to free-space transmission gratings. They were conceived largely as a high-channel-count alternative to daisy-chained FBGs for “plug and play” wavelength separation. Practicality issues such as manufacturability/yield/cost, thermal stability, and package integration for routing devices have thus far limited widespread deployment of AWG-based solutions.
Most high-channel-count products being developed and deployed today are designed around free-space diffraction gratings because of an optimum combination of cost and performance. VPH transmission gratings are being chosen more and more as the advantages of transmission geometry, dispersion, efficiency, PDL, scatter, and ruggedness come to light.
Figure 2. Qualitative comparison of wavelength selective technologies.
Without getting into the mathematical details of coupled wave electromagnetic theory of periodic structures, we outline a few basic principles regarding VPH transmission gratings:
- They are periodic phase structures in optically thick films, that is, several wavelengths deep. Phase is modulated by the hardness, or refractive index, of the film. Key physical characteristics of a VPH grating are a) the period or distance between fringes, b) the amplitude of the refractive index variation across that period, and c) the thickness through which that periodic index variation exists
- The diffraction efficiency (DE) of a VPH grating depends on the above key physical characteristics, along with d) the wavelength, e) the polarization state, and f) the angle of illumination.
- The efficiency of a VPH grating normally optimizes at the Bragg angle or Bragg condition. Restricting our attention to VPH gratings in which the fringe plane are parallel to the surface normal, this special case is also the angle at which the first diffracted output order is at an angle equal and opposite the surface normal to the input angle. This is directly equivalent to the well-known Littrow condition for a reflection grating (which renders the output an often-inconvenient retro reflection of the input). The terms Bragg and Littrow are often used interchangeably in reference to VPH transmission gratings. The angle at which this condition is satisfied is a simple function of the wavelength of illumination and spatial frequency of the grating.
- Assuming illumination at the Bragg angle, the efficiency of the grating increases as we increase from zero either the amplitude of the index modulation, DN, or the thickness though which that phase modulation exists, T. To very close approximation, the efficiency is simply a function of the product DNT of the modulation amplitude and the film thickness.
- The peak efficiency at the Bragg condition of a VPH transmission grating does not saturate at 100%, but rather is periodic with DNT. Furthermore, the period of that DE vs. DNT behavior is different for S and P polarizations. The relative periods of the S and P curves are determined by the angle between the input and output coupled waves inside the grating volume, with the period of the P curve always longer than that of the S curve.
- When the coupled waves are separated by 90° inside the grating, the P period is infinite – that is, P efficiency is everywhere zero. There are two unique operating angles, on opposite sides of this 90° singularity, that result in a P period exactly three times the S period, as shown in Figure 3. When designed for operation at one of these two unique angles, the first peak of the P curve exactly overlays the second peak of the S curve, resulting in theoretically 100% diffraction efficiency in both polarization states.
Figure 3. UltraSpec™-HT low PDL VPH grating design point.
Hence VPH transmission grating technology offers readily accessible and manufacturable configurations that provide high, polarization-independent efficiency along with high dispersion. For the telecommunications C-band centered at 1545 nm, the two design points correspond to grating frequencies of 940 lines/mm and 1350 lines/mm. The principle may be applied at any wavelength, with the grating frequency scaled accordingly.
The UltraSpec™-HT940 is a 940 line/mm VPH transmission grating design for telecommunications employing the lower of the two unique coupled wave angles. It maintains high efficiency over the entire C-band wavelength range, as shown in Figure 4. The design incidence angle is 46.56°. As noted above, this incidence angle is, to first order, independent of design wavelength. Dispersion of this product is 0.078°/nm, as shown in the output angle vs. wavelength curve of Figure 5.
Figure 4. UltraSpec™-HT940 typical measured efficiency
The UltraSpec™-HT1350 is a 1350 line/mm VPH transmission grating for telecom C-band that is laminated between a pair of prisms. The prisms are necessary to access the higher of the two unique internal coupled wave angles discussed earlier, as that angle is beyond the critical angle of the material refractive index. Such a lamination of a grating and prisms is often referred to as a grism. As the wavelength departs from the Bragg condition, decoupling of the P-polarized efficiency curve happens more rapidly for the higher angle 1350 line/mm design than for the 940 line/mm design. This is seen in the measured efficiency vs. wavelength curves of Figure 6, in which P-polarized efficiency falls to a still-respectable 80% at the edges of C-band. For some applications this may be a reasonable price to pay for the higher dispersion.
The use of prisms adds an additional degree of flexibility to the design, in that the output prism angle can be customized to impart a Snell’s law “magnification” to the grating dispersion. One standard version of the UltraSpec™-HT1350 uses 45° prisms, and provides a dispersion of 0.108°/nm, as shown in Figure 7. A significantly higher dispersion of 0.182°/nm is provided using an output prism angle of 83°, as shown in Figure 8. Any dispersion value in between can be accessed through the selection of the appropriate intermediate prism angle.
Note that the use of an 83° prism on the grating output in combination with a 45° prism on the grating input imparts anamorphic magnification to the beam aperture. This can be an advantage or disadvantage, depending on the application. Suffice to say that the product characteristics are highly customizable. An 83° prism can be placed on both input and output to provide the same 0.182°/nm dispersion with no anamorphic magnification.
Figure 5. UltraSpec™-HT940 output angle vs. wavelength dispersion (0.078°/nm).
Figure 6. UltraSpec™-HT1350 typical measured efficiency.
Figure 7. UltraSpec™-HT1350 dispersion with 45° prisms (0.109°/nm).
Figure 8. UltraSpec™-HT1350 dispersion with 83° output prism (0.182°/nm).
If high efficiency and low polarization sensitivity are both required, then the optimum solutions for telecom will be embodied in the UltraSpec™-HT940 or one of the UltraSpec™-HT1350 products described above. However, there are applications in which efficiency may be sacrificed in return for increased wavelength separation. An example of such an application is the Optical Performance/Channel Monitor (OPCM), whose purpose is to monitor relative signal strength and noise levels between closely separated DWDM wavelength channels. They are used to monitor network integrity and provide the sensor function of a dynamic gain flattening control system.
An OPCM is not a device that is “in line” with the optical data transmission. Rather it monitors a sample of the optical data that is diverted from the main transmission path via a weak tap coupler. Even still, attenuators are usually placed between the tap and the OPCM in order to further reduce the tapped signal amplitude into the dynamic range of the detector, typically an InGaAs array. As such the efficiency of the path within the OPCM is far less important that it is within an in-line device, such as a MUX/DEMUX or a ROADM. However, PDL within an OPCM normally is a critical performance parameter. Furthermore, a grating with higher angular dispersion enables a more compact OPCM device.
VPH grating technology is well suited to address this application, using a very different design point optimized for dispersion and PDL, but not efficiency. The UltraSpec™-C160 and UltraSpec™-L160 are C-band and L-band versions of a prism coupled grating, or “grism” design that provides unprecedented angular dispersion of 0.30°/nm, as shown in Figure 9. This is nearly twice the dispersion of the UltraSpec™-HT1350 and four times that of the UltraSpec™-HT940. This dispersion is achieved using a significantly higher spatial frequency of 1790 lines/mm for C-band and 1742 lines/mm for L-band. Insertion loss is significantly higher, at nominally 4 dB, but very flat across the full extent of either band with nominally less than 0.2 dB PDL.
Figure 10 shows a schematic layout of an OPCM using the UltraSpec™-C160. Note how the high internal incidence angle at the grating plane provides a beam/grating footprint that is much larger than the cross section of the beam itself. This enhances achievable spectral resolution in a very compact space.
Figure 9. UltraSpec™-C160 with 45 degree prisms provides 0.30°/nm dispersion
Figure 10. Compact OPCM layout using UltraSpec™-C160.
The stability of the diffracted beam angle from a VPH transmission grating with temperature change is, as with conventional reflection gratings, a direct function of the thermal stability of the grating substrate. As the substrate of a grating thermally expands or contracts, the spatial period (1/frequency) of the grating expands right along with it. Therefore, zero expansion materials such as Zerodur are often used for reflection gratings.
However, the optical clarity and homogeneity of Zerodur is insufficient for use as an optically transmissive window. Zerodur is therefore unsuitable for use as the substrate and protective coverplate of a VPH transmission grating. The best currently-available VPH grating substrate, from the standpoint of high optical quality combined with low thermal expansion, is UV grade fused silica.
The thermal expansion coefficient (CTE) of fused silica is 0.52 x 10-6 / °C. This is more than an order of magnitude lower than an otherwise more economical high grade glass, such as BK7, which has a CTE of 7.1. If a VPH grating is to be operated over an extended temperature range, the material of choice for substrate and coverplate is normally fused silica. BK7 is a more economical alternative if higher spectrum drift with temperature can be tolerated or corrected elsewhere in the system.
If prisms are used with the grating to form a grism, the thermal stability equation changes considerably. Fused silica is a particularly bad material from which to build a prism for thermal stability. The thermal coefficient of refractive index dN/dT of fused silica is 12.8 x 10-6 / °C, more than an order of magnitude higher than that of BK7 glass, which is 0.9 x 10-6 / °C. Laminating fused silica prisms to a fused silica grating assembly more than offsets the thermal stability of the grating frequency. The thermal stability of an all-BK7 grism assembly is actually better than a construction of all fused silica.
The optimum grism construction for thermal stability assembles the grating film to a fused silica substrate, and uses a low dN/dT glass such as BK7 (or even BK1 at 0.1 x 10-6 / °C) for the prisms.
The major challenge in building a grism from these dissimilar materials is to hold them together over wide temperature ranges such as the –40°C to +85°C temperature cycling requirements of Telcordia GR-1221. The usual techniques of optical doublet bonding either delaminate the bond or fracture the materials themselves when subjected to such a wide range. Kaiser Optical Systems has developed a patent-pending grism lamination process that maintains full integrity over these temperature ranges.
The UltraSpec™ products described above are available in configurations that have passed full Telcordia GR-1221 environmental qualification tests, including temperature cycling, temperature shock, high and low temperature storage, mechanical shock and vibration, and damp heat.
Damp heat testing is a subject that merits some discussion. The VPH grating medium is a gelatin film, which although otherwise very robust, is highly sensitive to moisture. A normally-laminated VPH grating, protected by a cover glass, an optical adhesive, and a small gelatin-free border, would last forever in any normal earthly environment.
However, in order for a standalone UltraSpec™ grating to independently pass the most demanding GR-1221 damp heat cycles (not a normal earthly environment), a rigorous hermetic seal is required. Such as seal must be, unlike any optical adhesive available today, a complete barrier to water vapor. The best optical adhesives available, whether UV-cure or thermal cure, are very good at preserving clear, high integrity bonds through all GR-1221 cycles including damp heat. But all transmit some level of water vapor to the VPH grating gelatin layer within prior to completing the prescribed exposure.
Kaiser Optical Systems has developed and qualified an optional soldered-edge process for UltraSpec™ products that protects the grating film from all levels of GR-1221 damp heat exposure. The expense associated with this process has been deemed unnecessary by some users of this product, in particular those who package the UltraSpec™ grating or grism in a hermetic enclosure.
As described earlier, a requirement for low PDL in combination with high efficiency and substantial dispersion limits a telecom application to two basic VPH component configurations, the UltraSpec™-HT940 grating or the UltraSpec™-HT1350 grism (with variants based on prism angle.) However, if an application requires diffraction efficiency in only a single polarization, a much broader grating design space becomes accessible.
High efficiency can be achieved for a single polarization at nearly any spatial frequency for which the desired operating wavelength diffracts to a propagating order. S polarization (TE) is generally preferred, because it can generally maintain high efficiency over a much wider range of wavelengths and/or input angles. This is evident when comparing the S and P curves in all of the previous examples. However efficient P-polarized (TM) gratings for more limited operating wavelength ranges may also be constructed at nearly any spatial frequency, as long as they are not too close to the 90° internal coupled wave angle singularity discussed earlier.
An example of an effective use of single polarization design is shown in Figure 11. This is a modified version of the UltraSpec™-HT940, having very different film characteristics for optimization of S polarized bandwidth. Unlike the low-PDL version, this version maintains reasonably high diffraction efficiency in S-polarization over all optical telecom bands from1300 to 1600 nm.
Figure 11. Special 940 l/mm design with high S-polarized DE from1300-1600 nm
Even in applications that prefer the reflective layout of a conventional reflection grating, the VPH transmission grating is worthy of consideration. A VPH transmission grating in combination with a flat mirror may mimic the form, fit, and function of a reflection grating. This is shown schematically in Figure 12.
Figure 12. Double pass VPH grating doubles dispersion and mimics reflection grating.
Note that the angular wavelength dispersion is doubled on the second pass, such that a given frequency VPH transmission grating functions in double pass as a reflection grating of significantly higher spatial frequency. For example, a 940 line/mm VPH grating in double pass has the equivalent C-band dispersion of a 1170 line/mm reflection grating, or 0.157°/nm. However the Littrow incidence angle on the double pass 940 grating is only 46.6 degrees, compared to the much higher Littrow angle of 64.7 degrees on the 1170 reflection grating. This reduced obliquity can provide more compact packaging and performance advantages.
Performance of a double pass geometry is normally optimized if the mirror is aligned to retro reflect the center wavelength of the intended operating range. This places the “off-Bragg” condition of the extreme wavelengths at equal magnitudes, which generally produces the best balance of P-polarized falloff. Note that the off-Bragg magnitude of the 2nd pass is larger than that of the first pass, such that the total insertion loss away from the center wavelength is worse than a simple square of the single pass performance. Figure 13 shows measured double pass performance of an UltraSpec™-HT940 grating in this orientation.
Figure 13. Measured double pass performance of UltraSpec™-HT940.
All of the performance curves presented in the discussions above assume fixed input angles at the Littrow, or Bragg condition for the center wavelength in the intended operating range. However, there are some diffraction grating applications that use the grating as a wavelength tuning mechanism. Such applications would include, for example, tunable lasers or tunable bandpass filters. In these applications, a conventional reflection grating is tilted to Littrow for the wavelength to be selected, such that it retro reflects that selected wavelength.
A VPH transmission grating may be tuned in much the same way. Tilting a VPH grating to the Littrow or Bragg condition for a single wavelength of interest greatly broadens the operating wavelength range relative to a fixed incidence angle.
As a first example, consider a grating that we would like to operate over the full set of optical telecom bands, centered at 1450 nm. Using the UltraSpec™-HT design principles, we can build a high-efficiency, low PDL grating for 46.56° incidence at 1450 nm with a spatial frequency of 1005 lines/mm. Figure 14 shows how the efficiency of this grating falls with wavelength away from 1450 nm. At the fixed input angle of 46.56°, the falloff of efficiency is severe. However, if we tune the angle of the grating to Littrow for all wavelengths, the efficiency remains much more useable over the 1300-1600 nm wavelength range, particularly for S-polarization.
Figure 14. Improvement in wavelength range with angle tuning,
1005 line/mm HT design.
We showed earlier that, if we restrict our concern to a single polarization, we can also broaden wavelength range relative to a low-PDL HT design configuration. That concept can be applied in combination with angle tuning to generate extremely broad wavelength coverage. Consider again the case of a device for operation over all optical telecom bands, except now in S-polarization only, and with angle tuning. Figure 15 shows the performance of a 1012 line/mm VPH grating, optimized for wavelength range in S-polarization only, with the added improvement of angle tuning.
Performance of this configuration is extremely broad, easily covering the full range of optical telecom wavelengths. Used in double pass, this angle-tuned 1012 line/mm VPH grating is an efficient alternative to a 1200 line/mm reflection grating, having the same high dispersion of 0.187°/nm. For a given beam size, the grating footprint and resulting grating size is much smaller on the 1012 VPH grating vs. the 1200 reflection grating, due to the reduced incidence angle. This geometry comparison is shown schematically for a 1520-1570 nm laser tuning or tunable filter application in Figure 16.
Volume phase holographic (VPH) transmission gratings are powerful and flexible alternative to conventional reflective diffraction gratings for free-space wavelength separation in high channel count applications. The new UltraSpec™-HT line of gratings and grisms for optical telecommunication applications offers an unprecedented combination of efficiency, angular dispersion, and polarization insensitivity. These rugged, cleanable components enable the design of a wide variety of compact and high performance optical system configurations that would be otherwise unachievable. The physical properties of VPH gratings may be customized to access a wide design space of tradeoffs between performance parameters such as wavelength range, efficiency, PDL, and dispersion. Such customization is readily available at lower cost and shorter lead-time than customization of reflection gratings. VPH gratings are compatible with Telcordia GR-1221 environmental requirements, and are enjoying increasing and enthusiastic acceptance and deployment in metro and long haul DWDM networks.
Figure 15. Extreme wavelength range using single polarization
design plus angle tuning.
Figure 16. 1520-1570 nm Laser Tuning Geometry Comparison