The Advantages and Challenges of Single-Mode Semiconductor Lasers

Single-mode semiconductor lasers provide an inexpensive and compact laser source for a wide range of applications, including Raman spectroscopy, Raman microscopy, process Raman, metrology, atomic clocks, interferometry, and cutting-edge quantum computing research.

While single-mode semiconductor lasers have various advantages, one disadvantage is their great sensitivity to optical feedback. Even small amounts of reflected light can have a destabilizing effect, such as frequency oscillations, mode hopping, intensity variations, and increased noise.

When a reflected beam re-enters the laser cavity, it contributes to the existing optical power. This can increase optical power density (W/cm² or W/m²) in the laser's active zone.

If the power density surpasses the laser's maximum damage threshold, it may result in thermal runaway, accelerated degradation, and, eventually, one of the most frequent causes of failure in single-spatial-mode diode lasers: Catastrophic Optical Damage (COD).

COD is defined as irreversible damage to the laser's active region, usually resulting in the loss of lasing capability and making the laser unusable.

The simplest technique to reduce these forms of back-reflections is to utilize an Optical Isolator, which functions as a one-way valve for light.

An optical isolator, created by cleverly combining optical polarizers and waveplates, enables efficient (e.g., >92%) forward light transmission while effectively preventing light transmission in the reverse direction, with attenuation of >33 dB.

Optical isolators significantly increase the lifespan of single-spatial-mode laser diodes by removing the risk of unintended back reflections.

In addition to preventing COD, optical isolators can increase the performance of single-mode semiconductor lasers by eliminating the negative effects of back reflections.

By preventing reflections from re-entering the laser cavity, the isolator reduces disturbance to the laser's output power, frequency, and beam quality. This results in increased laser stability, wavelength control, and overall performance.

The table below summarizes the numerous optical isolators that can be used in IPS lasers, as well as their typical isolation and insertion losses.

Source: m-oem

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This information has been sourced, reviewed and adapted from materials provided by m-oem.

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