PRACTICAL OVERCOMING OF SOLID STATE LASERS CONS

Аннотация

In this article, we explore drawbacks of solid state lasers and compare practical solutions for them: namely, thermal stress and overheating, electrically caused damage. A numerical experiment was set up using software package Cadence OrCAD, during which the current-voltage characteristics of a low-power laser diode were measured. A brief description of the accompanying physical processes and consequential effects are given for clarity. Practical advices for such problems described and given in detail. 

Полный текст

Lasers work on so called population inversion – in idle state, electrons in gain (or active) medium are mostly present at lower energy levels, as it is natural for them to seek lower energy state. When we apply pumping energy to medium, electrons move to higher energy levels using this pumped in energy. But energy states of electrons in atoms are discrete, and when electron is moving from higher to lower energy level, it releases energy equal to the difference of those energy levels.

The purpose of this paper is to explore and compare solutions for drawbacks of solid state lasers. In order to achieve it we conduct a comparative analysis of articles on the solving the problems of solid-state lasers. Relevance of this theme is present in fact that solid state lasers, (especially, semiconductor ones), are widespread and used in many areas. Highly scalable and cheap, they have good output characteristics.            

Thermal stress

When the nature of processes and properties of substances are related to the heat, a temperature is a physical parameter describing the energy state of the body. Heat may be one of the most limitations in increasing power of the laser systems.

Solids have much denser atom amount per same volume, thus, first advantage of this type of laser become present – lower volume of pump medium, while preserving amount of electrons used for emission - solid lasers are more compact.

When heating up, gain of solid state laser will withstand thermal stress caused by the thermal gradient ∆T is given by

where B is thermal expansion coefficient; E is Young’s modulus; and v is Poisson’s ratio.

It may cause thermal stress, stress birefringence and thermal lens effect which may degrade the optical properties of the laser medium, reduce the laser output and beam quality and even lead to medium break, so efficient heat removal is required. Furthermore, the temperature gradients generate mechanical stresses in the active medium since the hotter inside volume is constrained from expansion by the cooler outer zone. [1]

As a result, any conventional high-power solid-state lasers should be designed to operate in the presence of significant thermal loading of the gain medium. Even so, thermo-optic distortions usually limit a solid-state laser’s brightness and average power

Minimizing thermal stress

To operate within temperature specifications, virtually any high-power laser must be actively cooled, whereas many low-power lasers get by with passive cooling via either some sort of heat sink or just natural airflow. Laser diodes (LDs) are a special case: their small size means that even low-power emitters require a well-thought-out cooling configuration [2, 3]

For the lowest-power LDs (and many pulsed LDs), conductive cooling by itself is adequate. Higher power LD may require the addition of a thermoelectric cooler (TEC). For the highest output powers, such as those produced by diode bars and arrays for materials processing, water cooling is necessary.

Many critical laser-diode parameters such as wavelength, threshold current, efficiency, and lifetime are highly dependent on junction temperature. As an example, laser diodes may be rated for 100,000 hours running at 25°C, but would only run for 10,000 hours at 55°C. [5, 6, 7]

This is why highly stable temperature control is a desirable aspect of any setup.

Typically, the cooling system should dissipate 15-20% more power than the laser generates - to compensate for dust build-up, deterioration of fan ball bearings, and so on. Consequence of inadequate laser-diode cooling may be damage to diode.

Electrically caused damage

The second damage mechanism is related to failure of a laser diode’s P-N junction itself. A severe over-current or over-voltage power surge can cause localized heating and other harmful phenomena, which, under extreme conditions, can cause fracture. Current vs voltage profile of typical laser, result of numerical experiment, is shown below. (Fig.1)

Figure 1. Current vs. voltage profile of a typical low-power laser diode [4]

Starting from zero volts, very little current flows until around 0.4 volts is reached. Further incremental positive increases cause current flow to increase at a roughly exponential rate. However, the laser diode does not emit laser light until the current exceeds a “lasing threshold,” which, here occurs at around 30 milliamps and at around 2.2 volts. With further incremental positive increases in voltage, current flow continues to increase, while the optical power emitted by the laser diode increases at a rate that is roughly proportional to current.

Once the maximum design current for a particular laser diode is reached (which is around 45 milliamps and 2.8 volts for this laser diode), further increases in current will likely result in laser failure. Thus it is important to completely prevent voltage, and thus current, from increasing beyond the absolute maximum rating for a particular diode. In most cases, a low-power laser diode will be destroyed if the absolute maximum ratings are exceeded, even for a brief period of time.

Solution here is to precisely control input voltage and amperage by using special power units and current control, designed with LDs in mind.

Important design features when selecting proper current sources are shorting outputs to maintain output leads at identical potentials, slow start to protect against turn-on transients, independent drive current limits to prevent accidental current overdrive, over-voltage protection to prevent against voltage overdrive, and power-line transient suppression to protect from outside influences to devices. (Fig. 2) shows an example of a properly designed current source.

Figure 2. This properly designed current source. [6]

Laser diodes, like most semiconductor devices, can be easily damaged or destroyed by inadvertent electrostatic discharges (ESD). In fact, it’s been suggested that ESD is the single leading cause of premature laser diode failure. Since laser diodes can be damaged by voltages that are too small to feel through skin. Human skin is only sensitive to 3000V or more, yet empirical evidence has shown that commercially available InGaAsP and AlGaAs laser diodes can be damaged by ESD voltages as low as 1200V. Just because you don’t hear or feel a static spark, don’t assume there isn’t a dangerous ESD discharge.

 As with all semiconductor devices, ESD has the potential for latent damage in laser diodes. In other words, ESD may simply weaken the device without any immediate symptoms. The static discharge breaks down the P-N junction in an area outside the optical cavity. During normal use, these defects propagate into the laser cavity over time. The resulting degradation in performance may appear long after the initial damage takes place.

In facilities where laser diodes (or other static-sensitive devices) are handled regularly, an ESD audit conducted by an independent certified consultant is recommended.

Whenever handling unprotected devices, wear a protective wrist strap designed to drain built-up electric charges safely to ground. Choose a secure, but comfortable strap with a 1 MΩ series resistor. Properly ground tweezers, soldering irons, and other tools as well. Also, it’s important to place unprotected lasers on static dissipative work surfaces. [6]

Difference in operation of damaged diode

  How can you tell if a laser diode is dead? A dead diode laser may still lase but will exhibit very stark performance issues from nominal. Strong reduction in output power, significant increase in threshold current can be signs that diode laser is dead. Laser focusing and collimation will also be affected when the laser chip has been damaged. The beam may diverge more quickly or will not be able to be focused to as tight of a spot as previously. If the laser has been damaged to the point where it will not lase, no light other than spontaneous emission will be observed. It’s impossible to fix a dead or broken laser diode.

Electrical damage has the potential to be latent. In other words, it may simply weaken the device without any immediate symptoms. The static discharge, for example, breaks down the P-N junction in an area outside the optical cavity. During normal use, these defects propagate into the laser cavity over time. The resulting degradation in performance may appear long after the initial damage takes place. When this “latent failure” finally occurs, it may be attributed to other causes.

Conclusion

  In conclusion, with proper and careful handling of solid state lasers, in properly equipped conditions, the problems associated with static electricity are solved. Electrical damage caused by too high current or voltage is solved firstly by checking and maintaining the power supplies, and secondly by protecting against voltage fluctuations in power grid even before the main power supply.

  Thermally caused damage can be solved by selection of adequate cooling according to the manufacturer's instructions, and in the case of experimental installations - calculation of the heat output, plus, deterioration over time of chosen cooling solution must also be taken into consideration.

  As with many semiconductor devices, electrical damage can be latent – without any immediate symptoms, but will surface over time. Damaged diode cannot be repaired.