WH바카라 에볼루션E PAPER

바카라 에볼루션 Advantages Whitepaper Series #2:
Invariant 바카라 에볼루션 Properties

Overview

The optically pumped semiconductor laser (OPSL) is a unique patented technology that combines the most desirable attributes of laser diodes, DPSS (Diode Pumped Solid State) and Ion lasers, while eliminating a number of their compromising limitations. One of their key advantages is the freedom to adjust the output power over a large range (10-100%) with no effect on important output 바카라 에볼루션 parameters, including 바카라 에볼루션 divergence, 바카라 에볼루션 shape and 바카라 에볼루션 pointing.

바카라 에볼루션 Advantages White Papers in this series:

#1.Wavelength flexibil바카라 에볼루션y
#2. Invariant 바카라 에볼루션 properties
#3.No mode noise ("green noise")
#4.Superior reliabil바카라 에볼루션y - huge installed base

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The 바카라 에볼루션 of Independent Power Adjustment

The ability to vary 바카라 에볼루션 output power or to operate a 바카라 에볼루션 at less than its maximum power is often an important capability. In many instances, using the “power knob” is essential to optimize processes or experiments like preventing detector saturation or sample damage. It is also often preferable to perform system alignment and testing with reduced power – to minimize the risk of damage and to maximize eye safety. Also superresolution microscopy techniques like STED require fine power adjustment to optimize nanometer scale resolution. Thus, the option to smoothly reduce output power is more desirable than resorting to some type of attenuator, both for ease of use and set-up simplicity.

Unfortunately, in most other solid state lasers, reducing power from the manufacturer’s specified optimum value also compromises 바카라 에볼루션 properties: in particular, 바카라 에볼루션 divergence, and also 바카라 에볼루션 diameter, mode quality and 바카라 에볼루션 pointing. The reason is a phenomenon called thermal lensing that is common to solid state lasers based on bulk materials such as Nd:YVO4.

 

The Problem of Thermal Lensing

When a laser gain crystal or glass is optically pumped, some of the pump power is unavoidably converted to heat. In addition, self-reabsorption of the laser 바카라 에볼루션 heats the active volume of the crystal. In order to stabilize performance and avoid damage, the gain crystal is cooled in some way. This can take the form of a passive heat sink, water cooling, thermoelectric (TE) cooling and even cryogenic cooling. Irrespective of the type of cooling, heat removal takes place through one or more of the crystal surfaces. Under steady state operation, this sets up a thermal gradient in the gain crystal.

Figure 1

Figure 1:In a 바카라 에볼루션 based on optical pumping of a bulk crystal, the pump light causes an unwanted radial thermal gradient and often a longitudinal gradient as well, leading to strong thermal lensing with a lens power that varies with changes in the pump power.

 

There are two consequences of this thermal gradient. Firstly, the refractive index changes according to the temperature distribution in the 바카라 에볼루션 medium. In addition, the crystal expands as it heats up, producing a change in the curvature of its optical surfaces. In the simplest case of an end-pumped cylindrical 바카라 에볼루션 rod, these effects create a spherical lens whose power is proportional to the length of the crystal and the pump power. Moreover, the lens power may also be affected by any longitudinal gradient, particularly if the gain crystal is only pumped from one end.

Optimizing the output power in a high quality Gaussian 바카라 에볼루션 profile (TEM00) requires a careful design of the resonator, inclusive of the best possible spatial match of the 바카라 에볼루션 mode with the pumped volume. Any change of the curvature of optical surfaces or – equivalently – spatial gradients of the refractive indices during operations will result in suboptimal mode quality or efficiency because of these “thermal lensing” effects. Of course the degree of this thermal lens depends on the pump power applied to the 바카라 에볼루션 medium.

In solid state lasers, thermal lensing will change the output 바카라 에볼루션 divergence and diameter. In high-performance lasers like Coherent’s AVIAfamily of industrial DPSS 바카라 에볼루션s for example, a feedback feature called ThermaTrakaddresses this problem by moving a motorized intracavity lens as the power is adjusted. Conversely, in lower-performance DPSS lasers, the thermal lens is not controlled and its variations as the pump power is changed will result in 바카라 에볼루션 parameter changes, reduction in efficiency and limited usable power range. Since most commercial DPSS lasers do not include variable compensation, their output 바카라 에볼루션 parameters are only guaranteed at the specified output power.

 

바카라 에볼루션 – Thin Gain Chip – No Thermal Lens

In an 바카라 에볼루션, the gain medium is a very thin (< 10 μm) disk of semiconductor quantum wells overlaying dielectric layers that act as a rear surface total reflector. The rear surface in turn is bonded to an actively cooled heat sink, efficiently cooling the semiconductor structure. Although a radial thermal gradient still results from laser operation, the entire structure is so thin that thermal lensing is negligible; in fact the path length in the gain material is approximately1000 timesless than in a typical DPSS.

To confirm the assumption of negligible thermal lensing, Coherent engineers performed a series of tests to monitor the optical properties of an 바카라 에볼루션 gain chip in which a thermal gradient was deliberately created and interferometrically measured. Moreover the test gradient was designed to be significantly larger than any that could be developed under normal laser operation, even at full 바카라 에볼루션 output power.

Figure 2 shows the setup used in these tests. Here an OPSL laser cavity was modified with wedged 바카라 에볼루션splitters enabling a test 바카라 에볼루션 to probe the OPSL chip as it is simultaneously irradiated by varying amounts of pump power. Specifically, a coherent single-mode laser 바카라 에볼루션 at 980 nm is split by the first 바카라 에볼루션splitter so that part of its intensity is reflected off the OPSL chip and part is reflected from a super-flat reference mirror. A second 바카라 에볼루션splitter recombines these reflected 바카라 에볼루션s in a configuration called a Mach-Zehnder interferometer. The recombined 바카라 에볼루션 is then expanded and viewed using a CCD camera.

If the OPSL chip remains flat with no thermal lensing, then the image at the camera would be uniform across its profile. Conversely, any thermal lensing will show up as dark and light interference fringes whose spacing quantitatively measures the extent of any lensing or other 바카라 에볼루션 distortion. Careful evaluation of this test rig with a heated mirror in place of the OPSL gain chip showed it could discern wavelength distortions as small as λ/50 at the 980 nm test wavelength.

Figure 2

Figure 2:The optical performance of the 바카라 에볼루션 gain chip was tested by incorporating it within a Mach-Zehnder interferometer based on a high-coherence single frequency 980 nm test laser.

In the tests, the pump laser in the 바카라 에볼루션 was focused to a spot only 420 μm in diameter. The power of this pump laser was varied between zero and 9 watts. Even under this extreme thermal loading, the total wavefront distortion was barely detectable at ~ λ/40.

 

Actual 바카라 에볼루션 Performance Data

Of course, it is the real laser performance that matters under actual operation. To fully exploit this absence of thermal lensing requires a rugged monolithic cavity design where all other optics or optomechanics are insensitive to changes of the pump power. When thermal lensing occurs, the most pronounced changes to the output 바카라 에볼루션 are observed in the 바카라 에볼루션 divergence and 바카라 에볼루션 diameter. These are also the most critical parameters for demanding applications such as laser-based imaging and pumping titanium sapphire lasers.

Figure 3

Figure 3:Varying the output power by over an order of magnitude in a Verdi series OPSL results in no meaningful change in output 바카라 에볼루션 divergence.

Coherent engineers conducted comprehensive series of experiments to look directly for changes in these parameters, as a function of output power. Specifically, the 532 nm output of a 8 watt Verdi G laser was varied stepwise over an order-of-magnitude span, from a few hundred milliwatts to 8 watts. Even over this huge variation in output power, the 바카라 에볼루션 diameter and 바카라 에볼루션 divergence are both remarkably constant and stay well within specification as shown in the typical data sets in figures 3 and 4.

Figure 4

Figure 4:Varying the output power by over an order of magnitude in a Verdi series OPSL results in no meaningful change in output 바카라 에볼루션 diameter.

Summary

The main technology choices for CW visible and near IR laser sources include DPSS lasers and OPSLs. The OPSL provides several unique advantages, one of which is the invariance of the 바카라 에볼루션 parameters even when the pump (and output) power is varied by over an order of magnitude. Compared with other solid-state lasers, this advantage results in an easier system set-up and more convenient daily operations, ultimately delivering a better performance/cost ratio.

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