• What is a RW laser?
• What is a RWE laser?
• Why do I need an external cavity for a RWE laser?
• What is a Broad Area Laser?
• What is a tapered amplifier?
• How can I adjust the TPA to reach maximum output?
• What is a tapered laser?
• What is a DFB laser?
• How can I use the thermistor of the TOC03 packages?
• What is the real linewidth of eagleyard’s DFB lasers?
• What is a DBR laser?
• What influences the life time of a laser diode?

RW Laser

What is a RW laser?

RW is an acronym for ridge waveguide. The top of the RW laser has an etched ridge which causes a step of the refractive index. This structure forms a waveguide that confines the light laterally.

The RW laser is an edge-emitting Fabry-Perot laser. The laser chip has facets cleaved to form flat and parallel mirrors at the front and at the end. The waveguide together with the mirrors builds an optical cavity.

RWE Laser

What is a RWE laser?

A RWE laser is a modified RW laser (see RW laser) for operation in an external cavity. Whereas the cavity of the RW laser is formed by the front and end facet of the laser chip the front facet of the RWE laser has an anti-reflecting (AR) coating. This enables the RW laser to be operated within an external cavity (see external cavity laser). An additional tuning element inside the external cavity allows the selection of the laser wavelength within the gain spectrum of the RWE laser.

Why do I need an external cavity for a RWE laser?

The RWE type (see RWE laser) is not a complete laser and has to be operated with an external cavity. At least it has to be completed with a collimating lens and an external mirror.
If you like to build a tunable external cavity diode laser, you have to add a diffraction grating. The Littrow configuration and the Littman-Metcalf configuration are common setups for tunable laser diodes.

We can't guarantee a minimum output power because the output power depends strongly on the losses of the external resonator.

But by a rule of thumb: If the losses are kept low and the external resonator is well adjusted, it is possible to achieve 50 % of the output power of the similar laser with standard coating.

BAL

What is a Broad Area Laser?

A Broad Area Laser (BAL) is an edge-emitting laser with a very broad active region. Stripe widths from 60 µm to 400 µm enable the high output power of BALs up to 12 W. The Broad Area Lasers operate spatially and longitudinally multi mode.

TPA

What is a tapered amplifier?

The tapered amplifier comprises an index-guided straight section and a gain-guided tapered section. Both facets are AR coated.

The tapered amplifier can be seeded by a DFB, DBR or a RWL laser. Also an external cavity laser with a RWE laser can be used as a seed laser. The emission of these lasers can be amplified if the laser wavelength of the seed laser is within the gain profile of the tapered amplifier. Maximum output powers range from 0.5 to 2 W.

The seed laser – also called master laser – is coupled into the narrow straight section. The amplified laser light is emitted at the facet of the tapered section. Because both sides have to be accessible, eagleyard provides the tapered amplifier with a C-mount package.

How can I adjust the TPA to reach maximum output?

As a manufacturer of laser components we are not able to provide detailed information of the setup of the system. But let us try to give you some basic hints that hopefully will help to find an appropriate setup.

Important for the laser system is a good match between seed laser and the ridge waveguide section of tapered amplifier.  We achieve the best results by seeding the TPA with our own lasers. They have nearly the same size of the active region. Therefore a 1 to 1 magnification is sufficient for an efficient coupling. The launched power of the seed laser should be in the range from 10 mW to 50 mW. The exact focussing into the active region of the tapered amplifier is very important in order to achieve the maximum output power of the tapered amplifier.

We recommend to start the adjustment by disconnecting the power supply from the TPA and using the TPA as a photo detector. A photo current of 5 to 6 mA indicates a sufficient optical coupling.

Afterwards you should start with a current of 500 mA and optimize the adjustment of the optical coupling. Next step is to increase the current by 500 mA and readjust the coupling again. This procedure should be repeated until you have reached the desired power.

TPL

What is a tapered laser?

The tapered laser (TPL) combines the good beam quality of a RW laser (see RW laser) with the high output power of a broad area laser (see Broad area laser). The design of this laser is very similar to a tapered amplifier (see Tapered Amplifier). As the tapered amplifier it consists of an index-guided straight section and a gain-guided tapered section. But whereas the tapered amplifier is AR coated on both facets the TPL has a reflection coating on the facet of the straight section. This feedback determines the laser wavelength.

DFB Laser

What is a DFB laser?

A distributed feedback (DFB) laser comprises a periodic structure close to the p-n junction of the diode. The distributed feedback from this grating determines the emission wavelength.

The DFB laser has a very stable wavelength which can be slightly tuned by the temperature and the current of the laser. The wavelength coefficients are 0.06 nm/K and 0.003 nm/mA.

How can I use the thermistor of the TOC03 packages?

The TOC03 packages of our DFB and DBR lasers comprise a thermistor that is located close to the laser chip. The thermistor enables the measurement of the laser temperature. For many applications it is important to control the wavelength by stabilizing the temperature of the laser.

The integrated thermistor has a negative temperature coefficient (NTC). The resistance decreases with increasing temperature. eagleyard uses NTCs from Betatherm, Type 10K3A1 (10kOhm @ 25°C, typ. Beta = 3892).

You will find a good explanation of the Beta Coefficient and a table of the resistance versus temperature on the web site of the manufacturer.

What is the real linewidth of eagleyard’s DFB lasers?

We specify a maximum linewidth of 10 MHz. In our lab we could measure linewidths between 2 to 4 MHz. This measurement was limited by the influence of the laser driver and vibration of the building.

Some of our customers have the ability to perform the measurement with higher accuracy. They reported linewidths below 1 MHz.

DBR Laser

What is a DBR laser?

The distributed Bragg reflector (DBR) laser is very similar to a DFB laser (see DFB laser). Whereas the DFB laser has a grating that covers the whole active region of the laser chip, the DBR laser consists of three different sections: a gain section, a phase section and a Bragg reflector section. Separated contacts of the sections allow the individual control of gain, phase shift and Bragg reflection.

As the DFB lasers the DBR lasers provide a single mode emission at a precise wavelength with an extremely narrow linewidth.

Life time

What influences the life time of a laser diode?

Thermal stress can induce mechanical stress due to the different temperature coefficients of the components. This can cause the growth of crystal defects or put a strain on the connections inside the laser device. Both effects can shorten the life time of the laser device.

A momentary over current of the laser driver can damage the laser. A laser pulse with extremely high peak power and the high current concentration can destroy or at least predamage parts of the facet. Optical absorption at this defect can cause nonradiative recombination and lead to heating and melting. The higher temperature at the surface decreases the band gap leading to a current concentration at the defect. This increases the heating again and results finally in a catastrophic optical damage (COD).