Choosing the right laser for your holography research set-up

A well-chosen laser source can supercharge your holography research, but how do you select the right laser?

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Single frequency DPSS lasers facilitate precise and non-destructive testing of samples, materials, and structures. They play a crucial role in pushing the boundaries of holography research by providing the high coherence, stability, and precision required for a wide range of holographic techniques and applications. But which laser characteristics are the most important?

We've put together a streamlined guide to choosing the ideal lasers for your holography research. To optimise your holography research, check out these quick tips.

1. Coherence length

The coherence length of a laser defines the distance over which the light waves maintain a stable phase relationship. In holography, a high coherence length allows for the creation of larger holograms with a higher depth of focus, enabling the imaging of extended or three-dimensional objects.

2. Coherence

Laser coherence determines the depth and clarity of the holographic image. Single frequency DPSS lasers offer exceptional coherence that enables sharp and clear holographic images.

3. Spatial characteristics

Spatial characteristics, such as divergence and beam diameter, are also important. Low beam divergence is preferable to maintain parallel light rays, which helps in preserving the coherence of the beam over long distances. Larger beam diameters are often desired in holography as they can illuminate a larger field of view, leading to larger holograms.

4. Wavelength

The wavelength of the laser light is important because it determines the colour and the resolution of the hologram. Laser sources at 640 nm, 532 nm, and 473 nm are considered the optimal wavelengths for recording full colour RGB holographic images.

5. Laser power and stability

High power lasers reduce exposure times and increase the brightness of holograms. However, it's important that the laser power remains stable during the exposure time as fluctuations can lead to inconsistencies in the hologram. Continuous wave lasers are often preferred over pulsed lasers in holography due to their power stability. With their stable output power and frequency, DPSS lasers provide consistent and reproducible results, making them ideal for short and long-duration experiments.

6. Beam quality

The quality of the laser beam influences the quality of the holographic image. Ideally, a laser for holography should have a uniform intensity and phase across its profile, such as a Gaussian beam profile. The cleaner and more uniform the beam profile, the better the quality of the hologram.

7. Noise and vibration levels

Holography is highly sensitive to even minute environmental vibrations or acoustic noise, which can cause phase shifts leading to a blurry or distorted hologram. Hence, it's essential to use a laser source that has low noise levels and to set up the holography system in a vibration-free environment.

8. Compact footprint

Practical factors such as the size, portability, and ease of integration into your existing set-up are also important. The laser system should ideally be compact and easy to install. Additionally, it's worth considering the running costs, including power consumption and maintenance. A reliable, easy-to-service laser can save you a lot of time and resources in the long run.

In short, your chosen laser source should be highly coherent, exhibit a narrow linewidth, and operate on a single frequency. These attributes are crucial for creating high-definition, precise holograms. By considering these factors, you can ensure that you select the most suitable laser for your specific holography research – improving the reliability of your results and the efficiency of your research process.

But what about your holography set-up? Use these quick tips to help optimise your research:

1. Stabilise your set-up

Ensure your entire set-up – including the staging area, mounts, and other components – is free from vibrations. Vibrations can lead to blurring and distortion in your holograms.

2. Go high-res

Use a high-resolution plate that is designed for the wavelength of your laser. This ensures that the resulting hologram has captured the finest details.

3. Mirror, mirror

Ideally, your hologram subject should be as reflective as possible to your laser's wavelength. This enhances the brightness and clarity of your hologram.

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4. Hybrid holography

Integrate holography with other imaging methods – such as fluorescence, confocal microscopy, interferometry, and Raman spectroscopy – to obtain a comprehensive understanding of your holography subject.

A single-frequency laser is a type of laser which operates within a singular resonator mode, achievable when the net gain bandwidth of the laser remains narrower than the frequency spacing of the resonator modes [1]. In cases where the mode spacing isn’t adequately large to support a single resonator mode, single-frequency operation is maintained through the utilization of intracavity filters. This enables the single-frequency laser to possess an exceedingly narrow linewidth, frequently measuring only a few kHz [2], and an extended coherent length. A clear association exists between the coherence length and the spectral width of the laser [3].

Where λ0 the is the center wavelength of the laser, and the Δλ is its full width at half maximum (FWHM) intensity. Various designs for the laser source can achieve single-frequency operation, differing in terms of achievable output power, beam quality, noise level and compactness. They can be grouped in different types, such as single-frequency diode-pumped solid-state laser, fiber lasers, and single-frequency semiconductor lasers.

In DPSSLs, the resonator is pumped by diode lasers, which are compact and can efficiently pump if the absorption band of the solid-state gain medium aligns with their emission band.

Additionally, diode laser pumping offers a prolonged operational lifespan, typically around 20,000 hours, rendering it suitable for space-based applications. Since its initial demonstration in [4], neodymium-doped yttrium-aluminum-garnet (Nd:YAG) has emerged as the predominant gain medium in DPSSLs. The advantages of employing this gain medium stem from both the doping with Nd3+ ions and the YAG host.

Nd:YAG’s structure facilitates effective absorption of pump radiation due to its relatively narrow wavelength bands, particularly around 807 nm, which corresponds to an output of 1.06 μm. As a host material, YAG ensures high thermal conductivity and excellent optical quality [5]. Single-frequency DPSSLs are frequently developed in either longitudinal [6] or non-planar unidirectional ring resonator [7] configurations.

Figure 2. Typical configurations of diode-pumped soli-state lasers, (a) longitudinal resonator, (b) unidirectional ring cavity (Courtesy of Dr. E. Balliu, HÜBNER Photonics). 

These lasers can function either in continuous mode (CW) by employing a CW pump diode or in pulsed mode with Q-switching. Single-frequency lasing is accomplished through either a wavelength-dependent loss structure like a Fabry-Perot filter or Lyot filter, or through the Nd:YAG medium itself, as exemplified by the non-planar ring oscillator (NPRO) lasers. The latter, pioneered by Jane and Byer [8], stands as one of the most prevalent single-frequency DPSSLs, frequently utilized as a seed laser in fiber amplifiers, particularly for gravitational wave detection [9], owing to its extremely narrow linewidth (only a few kHz) and high stability. The output power range achievable by single-frequency DPSSLs typically falls below 3 W at nm.

The technology behind our single frequency operation

At HÜBNER Photonics we recognize the important role that linewidth plays when it comes to single frequency lasers. That is why the single frequency and single transversal mode operation of our lasers result in extremely narrow laser linewidths (<1 MHz specified and <100 kHz typically), low intensity noise of down to <0.1% RMS and a perfect diffraction limited TEM00 beam.

Another prominent feature of our lasers is the HTCure™ technology used to manufacture them. HTCure™ is a proprietary method for fixation of cavity components developed by Cobolt to ensure extremely high thermo-mechanical stability. During the manufacturing process we build our lasers into a hermetically sealed sub-package in a planar configuration. As part of our quality control process, the laser is baked up to over 100° C for several hours and at multiple phases to ensure it does not go out of alignment or sustain damage.

The HTCure™ technology addresses the issue of thermal and mechanical shocks or vibration that the laser might sustain over its lifetime, thus enabling us to offer a long warranty period to our customers.

Lastly, we also know that the choice of cavity design can affect the laser’s output characteristics. Our laser cavities include frequency selective optics which stabilize the resonator and prevent all but one longitudinal and transversal mode of the laser from being amplified in the oscillator. By combining advanced cavity designs, standing-wave as well as ring-cavities, with a thermo-mechanically stable platform, active temperature control of the complete cavity and robust fixation of miniaturized high precision optics, all our single frequency lasers provide very stable single frequency or single longitudinal mode operation over the whole laser lifetime and over a wide range of operating conditions.

Find the right single frequency laser for you

We pride ourselves in providing the broadest range of compact and extremely stable single frequency lasers. Over the years we have developed Cobolt lasers that are single frequency, as well as C-WAVE lasers which offer widely tunable continuous wave (cw) single frequency output based on optical parametric oscillator (OPO) technology.

The Cobolt lasers are all single frequency in the 04-01, 05-01 and 08-01 Series.

These lasers are by default also single longitudinal mode (SLM) lasers and are diode-pumped solid-state (DPSS) lasers, or as Cobolt have embraced: Diode-Pumped Lasers (DPL), all with intra-cavity nonlinear frequency conversion.

C-WAVE Series, also a single frequency laser, offers a wide wavelength range of tunable single frequency output from 450 nm up to 740 nm and into the NIR.

Both the Cobolt and C-WAVE lasers are suitable not only for advanced laboratory research but also for integration into analytical instrumentation for applications such as Raman spectroscopy, bioimaging, interferometry, holography and quantum technology research.

References 

[1] Yang, Z., Li, C., Xu, S., Yang, C. (). Fundamental Principle and Enabling Technologies of Single-Frequency Fiber Lasers. In: Single-Frequency Fiber Lasers. Optical and Fiber Communications Reports, vol 8. Springer, Singapore; https://doi.org/10./978-981-13--0_2.

[2] Bingkun Zhou, Thomas J. Kane, George J. Dixon, and Robert L. Byer, “Efficient, frequency-stable laser-diode-pumped Nd:YAG laser,” Opt. Lett. 10, 62-64 ().

[3] C. Akcay, P. Parrein, and J. P. Rolland, “Estimation of longitudinal resolution in optical coherence imaging,” Appl. Opt., vol. 41, pp. –, .

[4] M. Ross, “YAG laser operation by semiconductor laser pumping,” in Proceed- ings of the IEEE, vol. 56, pp. 196–197, .

[5] D.W.HughesandJ.R.M.Barr,“Resonantopticalsecondharmonicgeneration and mixing,” J. Phys. D: Appl. Phys., vol. 25, pp. 563–586, .

[6] T. Baer, “Large-amplitude fluctuations due to longitudinal mode coupling in diode-pumped intracavity-doubled Nd:YAG lasers,” J. Opt. Soc. Am. B, vol. 3, pp. –, .

[7] T. J. Kane and R. L. Byer, “Monolithic, unidirectional single-mode Nd:YAG ring laser,” Opt. Lett., vol. 10, pp. 65–67, .

[8] A. Buikema, F. Jose, S. J. Augst, P. Fritschel, and N. Mavalvala, “Narrow- linewidth fiber amplifier for gravitational-wave detectors,” Opt. Lett., vol. 44, pp. –, .

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