Gain chips are semiconductor optical elements used as the optical gain medium of external cavity laser diodes. Gain chips are used as a TLS (Tunable Light Source) that can change the oscillation wavelength using a wavelength selection filter such as a diffraction grating.
Example schematic of Littrow arrangement type external cavity laser
Gain chips have 2 opposing waveguide end facets, one with high reflectivity or a low reflectivity of several percent, and one with the reflectivity reduced as far as possible and optically coupled with a mirror or diffraction grating outside the element to comprise a laser resonator. Therefore, the waveguide is bent, AR (Anti Reflectance) film is attached to the end facet of the non-reflective side, and the waveguide is made diagonal to emit the light at an angle. This means that the spectral ripple can be kept low. Since one end facet is reflective, it is also called a Reflective SOA (RSOA).
Furthermore, a wide wavelength tunable range can be achieved with an optical gain range of 80 nm or above.
ASE Spectrum of Gain Chip
Anritsu has experience providing gain chips supporting external cavity lasers from the O-band to the U-band. In addition, either LR (Low Reflectance) type or HR (High Reflectance) type can be selected as the reflectivity of the perpendicular facet side, according to the optical system used.
Anritsu provides COC (Chip on Carrier) gain chips included on a compact carrier for installation to customer optical systems. All the COC type gain chips have burn-in (operation inspection before shipping) performed to provide extremely high reliability, and complies with the RoHS directive.
- Optical output: 40 mW or higher when external cavity oscillates (Note: The figure varies depending on the configuration of the external cavity.)
- Wavelength range: C-band, C+L-band
(from O-band to U-band are selectable)
- Low spectrum ripple
- COC type
External Cavity Type TLS (Tunable Light Source)
An external cavity laser diode has a resonant mirror on the outside of the semiconductor element, which is the gain medium. Furthermore, a light source called a TLS (Tunable Light Source) enables a wavelength selection filter, such as a diffraction grating, to shift the oscillation wavelength.
The waveguide inside a gain chip is diagonal on one end facet to reduce the reflectivity as far as possible. The opposite end facet is perpendicular and has an external resonance structure to change the reflectivity and perform oscillation via the light going back and forth between outside and the gain chip perpendicular end facet side.
There are 2 types of external cavity lasers using a diffraction grating; Littrow type and Littman type.
With the Littrow type, the primary diffraction light of the diffraction grating is directly fed back to the semiconductor laser to perform oscillation by resonating with the perpendicular end facet side with low reflection film (LR). Since diffraction is only performed once, greater optical output than the Littman type is obtained.
With the Littman type, the primary diffraction light of the diffraction grating is fed back inside the gain chip by reflecting it with a mirror to perform oscillation by resonating with the perpendicular end facet side with high reflection film (HR). Since diffraction is performed on the laser twice with diffraction grating, wavelength selectivity is high but output is limited.
Example schematic of Littrow and Littman type external cavity lasers
External cavity lasers are also used in the Integrable Tunable Laser Assembly (ITLA), which is the signal light source of an optical communication system.
ITLA internal structure schematic
Communication systems that exceed 100 Gbps use coherent optical fiber communications with ultra high-speed digital signal processing. Although it requires advanced integrated optics technology due to the complex configuration using polarization multiplexing and amplitude phase shift keying, there is demand for smaller digital coherent transceivers. This has led to a focus on silicon photonics technology, which uses a silicon chip including an optical waveguide, optical modulator, and wavelength filter. Silicon photonics uses a gain chip of Reflective SOA (RSOA) as the gain medium.
Silicon photonics schematic