Abstract
The gain-lever effect enhances the modulation efficiency of a semiconductor laser when compared to modulating the entire laser. This technique is investigated in a long-cavity multi-section quantum-dot laser where the length of the modulation section is varied to achieve 14:2, 15:1 and 0:16 gain-to-modulation section ratios. In this work, the gain-levered modulation configuration resulted in an increase in modulation efficiency by as much as 16 dB. This investigation also found that the 3-dB modulation bandwidth and modulation efficiency are dependent on the modulation section length of the device, indicating the existence of an optimal gain-to-modulation section ratio. The long cavity length of the multi-section laser yielded a distinctive case where characteristics of both the gain-lever effect and spatial effects are observed in the modulation response. Here, spatial effects within the cavity dominated the small-signal modulation response close to and above the cavity's free-spectral range frequency, whereas the gain-lever effect influenced the modulation response throughout the entirety of the response.
Highlights
This paper investigates the impact of different gain-lever configurations on the frequency response of a long-cavity multi-section quantum-dot semiconductor laser
The reconfigurable nature of the multi-section device facilitated the testing of extreme gainto-modulation section cases in an attempt to determine a relationship between modulation section length and enhancement of both the modulation efficiency and modulation bandwidth
Compared to the uniform biasing case, a 16-dB enhancement in the modulation efficiency was reported for a case where the small-signal modulated section was biased at threshold and the gain section was biased at an operating point with a high carrier density and negligible differential gain
Summary
This paper investigates the impact of different gain-lever configurations on the frequency response of a long-cavity multi-section quantum-dot semiconductor laser. The gain-levering of quantum-dot lasers is of particular interest due to their characteristically large differential gain under weak bias conditions (low carrier density), and saturated gain profile (negligible differential gain) under high current densities [7,8,9]. While previous studies of the gain-lever effect in quantum-dot lasers have utilized 4:1 gainto-modulation section ratios in short-cavity lasers (< 1 mm) [2], this work investigates an extreme gain-lever case, where the gain-to-modulation section ratio is 15:1 in a long-cavity laser (> 8 mm) These unique experimental results of representative extreme asymmetric bias cases (15:1 and 14:2) highlight the limit to the gain-to-modulation section ratio discussed numerically in [2] and compared with the single-section modulation response (0:16)
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