Output Coupler

What Does An Output Coupler Do?

Output coupler, or OC mirror, or partial reflector, is a semi-reflective and semi-transparent mirror used in the laser resonator, in order to extract the laser light from the laser cavity. An output coupler is often positioned at the end of a laser cavity. The main use of an OC mirror is to ensure efficient laser output at laser-specific or harmonic wavelengths. It achieves this by transmitting a controlled portion of the laser beam out of the resonator as useful output to achieve stable laser output, while simultaneously reflecting the remaining intracavity light into the laser cavity for circulation. An output coupler mirror sustains and amplifies the lasing process. What’s more, the applications of output couplers are versatile; the OC mirrors can be applied as partial reflectors to attenuate beams at a fixed ratio, or as a simple beamsplitter.

OC mirrors consist of optical glass substrates and dielectric coatings; the coating design is critical. In general, the most common optical coating design for output coupler is: The first surface (the surface towards the laser resonator) of the optical mirror is coated with semi-reflective coating (also called PR coating) that transmits and reflects certain percentages of of the laser wavelength, and the rear of the optical mirror is coated with anti-reflection coating (AR coating) to minimize unwanted reflections that could cause losses or interfere with the laser beam quality. 

One critical parameter that significantly influences the laser performance of an output coupler (OC) is its transmissity, which is the efficiency of tranmission of optical power. Typically, the OC transmissivity is engineered to maximize output power.

The selection of OC transmissivity directly influences:

1. The threshold pump power of the laser: A lower transmissivity means more light is kept inside the laser cavity, making it easier for the laser resonator to start oscillating;

2. Laser efficiencies;

3. Overall output power;

4. Stability of mode-locking and Q-switching

Therefore, selecting the right OC transmissivity requires careful balance based on the specific laser design and application.

Flat OC Mirrors and Curved OC Mirrors

The shape of the output coupler laser mirror can be flat or curved (often concave), depending on the application needs. 

Flat OC mirrors: Flat output coupler laser mirrors are preferred in laser designs where minimal beam divergence and simple alignment are desired. To explain further, a flat output coupler is typically used where stable optical modes (i.e., a beam that repeatedly bounces back and forth between the mirrors without diverging too much or collapsing) are supported by other curved mirrors within the cavity.

Curved OC mirrors: Curved (either concave or convex) OC mirrors are needed when additional optical focusing is essential within the cavity to ensure stable beam propagation and better mode confinement.

Shalom EO-Supplier of Output Couplers/OC Mirrors

Off-the-shelf and custom output couplers/OC mirrors are available in Shalom EO. We can provide OC mirrors of multiple shapes: including plano-plano output couplers, plano-concave output couplers, plano-convex output couplers, and wedged OC mirrors. The OC mirrors are suitable for various laser types, such as solid-state lasers, continuous wave (CW), and pulsed lasers. Our standard mirrors are manufactured using N-BK7 or Fused silica substrates, with dielectric coatings deposited on the substrates. The typical laser-induced damage threshold is 500mW/cm2@1064nm, 10ns, 10Hz. To help users find the partially reflective surface, our output couplers can contain arrow markings or indications to show the partially reflective surface.

A diverse selection of coating wavelengths is obtainable: 

Our off-the-shelf selections include:

Nd:YAG laser output couplers with various reflection/transmission ratios:

1. 532nm Nd:YAG laser output couplers (second harmonic generated Nd:YAG) for medical applications
2. 946nm laser output coupler (Nd:YAG laser)
3. 1064nm Nd:YAG laser output couplers (Nd:YAG fundamental);

Other laser output couplers with various reflection/transmission ratios:

1. 716nm laser output couplers
2. 808nm diode laser output coupler
3. 1030nm ytterbium (Yb) doped laser output couplers
4. 1645nm Er:YAG laser output coupler
5. ...and more custom wavelengths