Ultrafast lasers have transformed many fields, from femtochemistry and attosecond science to biomedical imaging, surgery, and industrial microprocessing. These systems demand components that maintain precise control over pulse duration, spectral profile, and wavefront while enduring high peak and average powers. 
One key enabling component is the diffraction grating, which is used for pulse stretching and compression in Chirped Pulse Amplification (CPA) systems. Though early systems tended to use ruled or dielectric surface relief gratings, the technology has migrated to the use of transmission gratings for higher performance, including Volume Phase Holographic (VPH) gratings. This white paper explores how Wasatch Photonics’ VPH transmission gratings offer distinct advantages in ultrafast laser amplifiers, including superior diffraction efficiency, high customizability, low scatter, and system compactness – all in a robust, manufacturing-friendly package.
Introduction to Chirped Pulse Amplification
Ultrafast lasers generate pulses on the order of picoseconds to femtoseconds (10-12 – 10-15 s), which are generated by syncing broad spectral bandwidths in phase to create short, intense pulses of light, and then amplifying them to achieve the high peak power desired for each application. Ultrashort laser pulses are typically amplified using a chirped pulse amplification (CPA) scheme, in which a femtosecond pulse is first stretched in time to reduce its peak intensity, then amplified, and finally recompressed to its original duration. This approach is used to prevent damage to optical components during the amplification process, thereby facilitating the generation of higher peak power.
A pulse stretcher—usually comprised of one or more diffraction gratings—is used to create the temporal dispersion needed to stretch the pulse before it is amplified. After amplification takes place, this is reversed in a pulse compressor with matched diffraction gratings, which returns the laser pulse to its original short pulse envelope.
Diffraction gratings used in pulse stretchers and compressors must meet demanding criteria: high diffraction efficiency across a broad bandwidth, low wavefront distortion, high laser damage threshold, and excellent spectral and wavefront performance. The choice of grating technology directly influences the efficiency, stability, bandwidth, and size of the laser system. For many years, metallic or dielectric surface relief gratings were the standard choice. However, volume phase holographic (VPH) transmission gratings have emerged as a high-performance, customizable solution with distinct optical performance, design, and manufacturing advantages. Wasatch Photonics was one of the pioneers of VPH grating technology and has been manufacturing VPH gratings for over 20 years. Our laser pulse compression gratings are used extensively in research labs, and in volume by commercial laser system integrators.
What Are Volume Phase Holographic Gratings?
VPH gratings are transmissive optical elements made by recording an interference pattern into a photosensitive medium—typically dichromated gelatin (DCG) or a photopolymer—hermetically encapsulated between two glass substrates. This pattern forms a sinusoidal variation in the refractive index across the volume of the recording layer.
Unlike surface relief gratings, which rely on etched grooves to diffract light, VPH gratings leverage Bragg diffraction, a volumetric effect where light is diffracted due to constructive interference within the material. This method offers several benefits, particularly for applications requiring high optical throughput, broad bandwidth, and precise control over dispersion. Additionally, when used in transmission, they allow the design of optical systems that are much more compact and easy to align.
Benefits of VPH Gratings in Chirped Pulse Amplification
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High Diffraction Efficiency
A primary advantage of VPH gratings is their capacity for exceptionally high diffraction efficiency, which is often greater than 95% in the first diffracted order when optimized, and >98% under the right conditions. This is particularly valuable in ultrafast laser amplifiers, where system efficiency directly affects the achievable pulse energy. Light can pass through the gratings several times in folded stretcher and compressor designs, so high efficiency is critical to minimize power consumption.
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- In pulse stretchers, high efficiency across a broad spectral range ensures that the stretched pulse maintains its energy and spectral integrity.
- In pulse compressors, efficient diffraction is crucial for restoring the pulse to its original temporal profile without excessive loss.
At Wasatch Photonics, we carefully engineer our VPH gratings to be optimized for each customer’s specific center wavelength (e.g., 800 nm for Ti:sapphire or 1030 nm for Yb-doped systems). We can also shape spectral response by adjusting line spacing and index modulation. We routinely hit >98% efficiency across the grating for higher spatial frequencies (1600-1800 l/mm), and can specify >92-96% for lower frequencies (800-1250 l/mm).
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Broad Spectral Bandwidth
Ultrafast lasers require gratings that can handle broadband spectra without introducing significant distortion or efficiency loss at the extremes of the operating band. VPH gratings offer tunable bandwidth capabilities by modifying grating thickness, input angle, and refractive index modulation. This is essential for:
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- Sub-30 fs pulse durations, which demand spectral bandwidths exceeding 30–50 nm.
- Tunable systems like optical parametric amplifiers (OPAs), where the gain bandwidth shifts depending on configuration.
- Near-IR ultrafast sources, where broadband operation is a key performance metric.
Additionally, VPH gratings offer customizable polarization control. Proper design can yield high diffraction efficiency for both s- and p-polarizations, though many pulse compression systems favor p-polarization due to mirror compatibility and damage threshold considerations. We routinely engineer single polarization designs in which almost all light is dispersed into the first order for that polarization, preserving precious power. Bandwidths of 14-200 nm are available, at wavelengths up to 2500 nm.
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Superior Beam Quality and Low Scatter
Because VPH gratings do not rely on surface grooves, they avoid the groove-ghosting and periodicity-induced scattering common to ruled or blazed gratings. The smooth, volume-based index modulation yields:
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- Minimal scattered light, enhancing signal-to-noise ratio in sensitive applications such as pump-probe experiments and nonlinear spectroscopy.
- Excellent wavefront quality, which is essential for maintaining diffraction-limited beam profiles throughout amplification and recompression. Both PV and RMS wavefront are typically specified: PV wavefront over the whole grating for alignment, and RMS for beam quality.
- Good spatial uniformity, which ensures that all areas of the beam are diffracted with the same high efficiency. The greater the uniformity, the larger the beam size can be, allowing a given grating to handle higher power.
To ensure high beam quality that is compatible with our OEM customers’ systems, we have established customized in-house test fixtures that ensure drop-in performance upon integration. The achievable wavefront performance depends on the aspect ratio of the part relative to the substrate thickness and thus can be engineered into the design. We routinely manufacture pulse compression gratings with dimensions ranging from 25-125 mm in size, and 4-10 mm in thickness.
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Compact, Transmissive Geometry
Traditional surface relief gratings used in pulse compressors often operate in reflection, requiring complex beam folding and large optical footprints. VPH gratings, by contrast, function in transmission, enabling more compact and modular chirped pulse amplifier designs. This has several practical benefits:
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- Smaller optical setups, crucial for laboratory-scale and portable ultrafast systems.
- Simplified alignment, as transmissive paths reduce the number of reflective surfaces and associated angular tolerances.
- Collinear propagation, which eases integration with fiber-based and diode-pumped systems.
At Wasatch Photonics, we have designed gratings for use in several geometries, including standard (Treacy) and bowtie configurations, with angles of incidence ranging from 24-68 degrees.
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High Laser Damage Threshold (LDT)
Modern VPH gratings using hermetically sealed DCG formulations can tolerate high laser fluences. While they may not yet match the extreme LDTs of dielectric surface gratings or etched gratings, VPH gratings offer sufficient power handling for many ultrafast amplifier systems. Several parameters relevant for gratings used in chirped pulse amplifiers include:
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- Laser power and repetition rate, as measured in J/cm2 and Hz, respectively
- Peak power density, as measured in W/cm2
- Average power density, as measured in W/cm2
Wasatch Photonics laser pulse compression gratings operating at 1030 nm, for example, typically meet ~25 mJ/cm2 (FWHM) for 250 fs pulses at a repetition rate of 100 kHz, and peak power densities of ~300 GW/cm2. For CW operation, average power densities of ~8 kW/cm2 are reasonable. Since every laser system has its own unique operating parameters, we actively work with our customers to test and improve LDT performance in each system.
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Ease of integration for greater manufacturability
The inherent robustness, durability, and transmissive nature of VPH gratings makes them much easier to work with in a volume manufacturing environment:
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- The lines in a VPH grating are hermetically sealed between two optical flats, making them easy to clean and handle. No special handling, training, or storage is required.
- Transmission gratings are easier to align than reflection gratings, and small misalignments tend to have far less impact on system performance.
VPH gratings are also highly durable and have widely proven field performance in lasers, analytical and medical instruments, in volume. Wasatch Photonics VPH gratings can be cleaned like any standard optical flat.
Use Cases and Application Scenarios
VPH gratings are increasingly being adopted in a range of ultrafast chirped pulse amplifier platforms:
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- Ti:sapphire amplifiers: Efficient compression at 800 nm with support for ps and fs bandwidths.
- Ytterbium fiber or thin-disk systems: Compact, efficient grating compressors for high average power at 1030 nm.
- Industrial micromachining systems: Rugged, sealed VPH modules used in compact industrial femtosecond lasers.
Their combination of efficiency, spectral flexibility, and mechanical compactness makes VPH gratings an ideal choice for both research and commercial amplifier systems. At Wasatch Photonics, we offer a selection of off-the-shelf gratings for online purchase and overstock gratings available by request to facilitate quick testing, as well as custom-designed gratings that are perfectly tuned to your requirements and delivered in volume.
Conclusion
As ultrafast laser amplifiers evolve to deliver higher pulse energies, shorter durations, and more compact footprints across a wider range of operating wavelengths, the supporting optical components must keep pace. VPH diffraction gratings offer a compelling blend of performance, efficiency, and versatility tailored for modern CPA systems.
With the ability to customize dispersion, support broad spectral bandwidths, and maintain high diffraction efficiency with minimal scatter, Wasatch Photonics’ VPH gratings have become the cornerstone of many chirped pulse amplifier designs.
We have the expertise in VPH grating fabrication you need to engineer your next ultrafast laser system, so contact us for your customized design today!
