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Doctoral Defense

GaSb-based Type-I Quantum Well Tunable Diode Lasers for Spectral Range above 3 µm

Meng Wang

May 4, 2018
2:30 PM
Light Engineering room 250
Advisor: Prof. Leon Shterengas

 

Compact and efficient spectroscopic sensors based on tunable narrow spectrum semiconductor lasers are in demand for industrial monitoring, medical diagnostics, home security, etc. Distributed feedback lasers are compact and robust but offer rather limited single mode tuning range of several nm near 3 μm. External cavity lasers can demonstrate order of magnitude wider tuning range. Further extension of the tuning range requires broadening of the gain spectral bandwidth. Often the gain broadening implies reduction of the peak differential gain and, thus, increase of the threshold carrier concentration and threshold current. Excessive threshold current limits the tuning range indirectly by restricting the range of the operating temperatures compatible with continuous wave (CW) regime. Increase of the number of active quantum wells (QWs) can balance the effect of the gain broadening on the threshold current. However, carrier transport constraints often restrict the number of QWs that can be efficiently pumped in diode laser heterostructures. Cascade pumping scheme solves the carrier transport issues and enables efficient interband laser operation with large number of active QWs. Moreover, in cascade laser heterostructures with slightly different QWs in each cascade the gain can be shaped to enhance the ECL tuning range. The number of stages required for an efficient operation for a given gain bandwidth depends on optical transition matrix element. The maximum values of interband transition matrix elements can be achieved in type-I QWs. This work focuses on the development of GaSb-based type-I quantum well (QW) cascade diode lasers emitting in spectral range above 3 μm and operating in continuous wave (CW) regime at room temperature (RT). Two-step and single-step etched narrow ridge devices were designed and fabricated to yield stable and efficient single spatial mode operation. Two-step etching design was demonstrated to suppress lateral current spreading in cascade laser heterostructures and yielded the minimal laser threshold. Various dry etching chemistries were tested. Straight ridge waveguide anti-reflection coated GaSb-based type-I quantum well cascade diode laser gain elements were used in Littrow external cavity configuration. The tuning range of more than 200 nm corresponding to the gain width at about 7 cm -1 level below peak was demonstrated. Bent ridge waveguide lasers were designed and fabricated to further suppress the internal cavity feedback and to extend the tuning range to the record high values above 400 nm from 2.8 to 3.23 μm.

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