Active Q-Switch Lasers

Deeper Dive into Actively Q-Switched Lasers

Peak Power and Average Power in ns and Sub-ns Lasers:

Necessarily, the smoother leading and trailing edges of a Gaussian pulse slightly reduce (by a factor of 0.94) the peak power for given pulse energy or average power. The point is that the situation might be further complicated when lasers emitting few-nanosecond-long or sub-nanosecond (e.g., picosecond) pulses come into the picture. Usually, a Gaussian pulse shape is an excellent approximation for the real pulse profile of Q-switched DPSS lasers, even though it is accurate only for lasers operating in single longitudinal mode (SLM). Most commercially available Q-switched lasers rely on multiple longitudinal modes, oscillating at the same time with constant interference. This effect is visible by acquiring optical pulses with sufficiently fast photodiodes and digital oscilloscopes, showing an amplitude modulation superimposed to the Gaussian pulse shape.

The period and depth of the modulation depend on the number of longitudinal modes oscillating together. For tens-of-nanoseconds-long pulses, resonators support so many modes that the modulation is averaged out and hardly visible. Still, for sub-nanosecond operation, only a few modes are left, giving rise to deep modulation on the pulse envelope and strong deviation from the ideal Gaussian shape. A full-width half-maximum pulse duration definition becomes questionable, and a more refined statistical definition should be employed, giving rise to a correction factor for the peak power higher than 1 (potentially a factor of 1.5 could be a reasonable guess).

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The Advantages and Disadvantages of Passive vs Active Q-Switching:

While there are a wide variety of q-switch technologies, notably including acousto-optic and electro-optic modulators, the technique as a whole can be broken down into two primary categories of q-switches,  passive and active.  Active q-switches rely on the ability of “flip a switch” and rapidly adjust the lasers cavity loss.  A perfect example of this is an acousto-optic modulator, which, when activated creates a grating which deflects many spontaneously emitted photons out of the laser cavity, but once the modulator is turned off it allows the spontaneous photons to pass through and initiate stimulated emission.  Passive q-switches, on the other hand, take advantage of a process known as saturable absorption.  Saturable absorption derives from the fact that there is a limit to the amount of light that a material can physically absorb at a given wavelength before the energy level become saturated and the material becomes temporarily transparent.  As a result, if a saturable absorbing material is placed inside of the laser cavity, it will absorb all of the spontaneously emitted photons until it reaches saturation where it will then allow the photons to pass through and initiate stimulated emission.

Both of these q-switching techniques produce short pulses and high peak powers, but they each have their pros and cons.  For example, active q-switching allows the user far more control over when the pulse will be emitted and therefore how long population inversion will be allowed to build up.  But, on the other hand many common active q-switches such as Pockels cells (which utilize the electro-optic effect) often require the driver to swing several kilovolts each time the switch is triggered.  By contrast saturable absorbers require no drivers at all, meaning that not only are they compact, but they are also far less expensive.   As a result, when you are deciding between an active versus passive q-switched laser there are four key things that you must keep in mind.

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Why is a Low Jitter Feature Important in Actively Q-Switched DPSS Lasers?

In actively Q-switched lasers, the user controls the pulsed laser output, so that no laser pulse emission occurs without providing a proper input signal, aka “the trigger”. Due to the trigger signal propagation through the interface electronics, the Q-switch driver chain, and the laser resonator build-up time, a time delay (Td) is present between the externally-supplied trigger signal and the actual laser pulse emitted by the laser source. The Td can show fluctuations if any electronics or optics involved in the pulse generation process have a functional variance in time.

The parameter Td is very relevant in the timing management of some applications. You must consider both the time delay (Td) and a time jitter (Tj), which is a statistical variation of the time delay depending mostly on:

• electrical noise in the trigger-chain
• pulse-to-pulse fluctuation of trigger-chain electrical parameters
• laser pulse build-up time-mechanism and associated fluctuations.
• fluctuation of rising (-falling) edge temporal profile of the trigger signal

Because of the jitter phenomena, the actual value of the time delay is statistically altered. Therefore, the laser pulse emission event happens (in the vast majority of cases) inside a normal time distribution, defined by an average time delay Td and a standard deviation value Tj (i.e 68.2 out of 100 pulses develop in the time interval Td ±Tj).

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High-Energy Q-Switched Lasers for Harmonic Generation – Part 1

Solid-state lasers offer many advantages over other laser sources. One of their most unique benefits is the ability to produce extremely high pulse energy.  Solid-state lasers can achieve this through q-switching, which is a process where the resonator losses are periodically increased, therefore interrupting the lasing process.  Unlike other laser pulsing methodologies, q-switching is unique in that the pump source remains active while no lasing is occurring.  Constant pumping forces the laser crystal to store the energy from the pump, similar to a capacitor. And just as with a capacitor in an electrical circuit, once you flip the switch and lasing can once again occur, the crystal will “discharge” all of the stored energy as fast as physically possible, resulting in an extremely high-energy laser pulse.

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High-Energy Q-Switched Lasers for Harmonic Generation – Part 2

In our last blog post (Part 1), we highlighted the benefits of high-energy q-switched lasers for harmonic generation.  In that article, we reviewed how q-switched lasers work and why the resultant short duration, high-energy pulses are required for higher-order harmonics. So, we won’t rehash that here.  Instead, we will highlight another set of lasers offered here at RPMC, which are ideal for higher-order harmonic generation, allowing it to produce wavelengths from the infrared to the ultraviolet; the Sol, Wedge, and Onda from Bright Solutions.  All three of these laser sources are robust and compact, making them ideal for integration into OEM systems.  In this blog, we will highlight several of the features which make each unit uniquely suited for harmonic conversion.

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Laser Requirements for Time Gated Active Night Vision Imaging Systems

That being said, in many active imaging systems, one is not merely interested in producing a two-dimensional image, but instead creating a three-dimensional “point cloud” of the environment. To produce such an image, the system must have a way of recording the round-trip time-of-flight (TOF) of the light. As a result, the camera and the illumination source must be pulsed, and the IR camera must be time-gated. This is remarkably similar to TOF lidar or TOF radar [4] with the exception that, instead of measuring the return signal with a single photodetector, the receiver is instead an infrared camera. While in theory this process could be done with an SLD, when you gain switch a diode source, the peak power of the laser is the same as in continuous operation, and as a result the pulse energy is typically rather low. By contrast, if a q-switched laser is used, short pulses can be generated with considerable peak powers. Since only a fraction of a percentage of the transmitted light from the IR illuminator will ever make it back into the imaging system, it is imperative that the pulse contains as much energy as possible to maximize the overall effacing.

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LIDAR Becoming the Future of Bathymetry:

While liquid water does have significantly lower absorption at these shorter wavelengths, that is not the only factor that will determine the lasers ability to penetrate deep below the surface of the water; scattering is also a significant factor.  Even if you measure crystal clear water, there will always be a certain amount of scattering associated with the Rayleigh effect.  As a result, it is essential to use a laser with significant pulse energy, for enough photons to survive the round-trip flight back out of the body of water without scattering or being absorbed.  Additionally, the laser must have as short a pulse width as possible since the depth resolution is directly proportional to the pulse width of the laser.  A detailed explanation of this relationship is beyond the scope of this blog post, but it should be reasonably straightforward to understand that the lidar system cannot resolve TOF less than the width of the laser pulse itself.

With all of these factors taken into consideration, it isn’t a surprise that very few lasers possess the pulse width, pulse energy, form factor, and active q-switching required for bathymetric lidar.

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Raman Spectroscopy: Why are Picosecond Pulses Superior to Femtosecond?

Raman-based spectroscopy, on the other hand, is an entirely different nonlinear technique, relying on the frequency shift experienced by laser radiation incident on a molecule, related to its rotational and vibrational modes. Not being related to electronic transitions, the Raman shift is relative concerning the irradiating wavelength, and therefore, unless pursuing coherent excitation, laser source tunability is not required. Raman-based spectroscopy, being a nonlinear process, usually requires ultrashort pulse generation (e.g., practical TPA set-ups usually require < 300 fs pulse duration). On the other hand, since the Raman gain is generally higher than TPA cross-sections, cheaper and simpler picosecond lasers could be efficiently employed in incoherent Raman spectroscopy. Furthermore, in Raman spectroscopy, the spectral resolution is related to the laser source bandwidth. Therefore, the narrower bandwidths of picosecond lasers represent a remarkable advantage over their femtosecond counterparts.

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