High-power quantum-dot tapered tunable external-cavity lasers based on chirped and unchirped structures

Abstract

A high-power tunable external cavity laser configuration with a tapered quantum-dot semiconductor optical amplifier at its core is presented, enabling a record output power for a broadly tunable semiconductor laser source in the 1.2 - 1.3 µm spectral region. Two distinct optical amplifiers are investigated, using either chirped or unchirped quantum-dot structures, and their merits are compared, considering the combination of tunability and high output power generation. At 1230 nm, the chirped quantum-dot laser achieved a maximum power of 0.62 W and demonstrated nearly 100-nm tunability. The unchirped laser enabled a tunability range of 32 nm and at 1254 nm generated a maximum power of 0.97 W, representing a 22-fold increase in output power compared with similar narrow-ridge external-cavity lasers at the same current density.

Unlocking Spectral Versatility from Broadly−Tunable Quantum−Dot Lasers

“…Another low threshold current density of 0.24 kA/cm 2 was reported in [35] and also in [7], despite both having thresholds of ~300 mA. These two devices were both tapered structures, having a narrow straight waveguide on one side, with a flared section ending in a much wider facet on the output side [7,35]. Tapered waveguides offer the benefit of high power previously only seen in broad area diodes, while keeping the nearly diffraction−limited beam quality of narrow ridge waveguides [48].…”

“…In fact, the smallest J th so far for a tunable QD laser was for [33], which despite having a moderately high I th of 170 mA, actually had a very low J th of 0.113 kA/cm 2 . Another low threshold current density of 0.24 kA/cm 2 was reported in [35] and also in [7], despite both having thresholds of ~300 mA. These two devices were both tapered structures, having a narrow straight waveguide on one side, with a flared section ending in a much wider facet on the output side [7,35].…”

“…Indeed, it is well known that an increasing level of inhomogeneous broadening leads to an increase in transparency and threshold current and a reduction of the modal and differential gain [18,19]. However, such inhomogeneous broadening and the resulting wider gain bandwidth can also be harnessed to great advantage in a range of devices such as mode−locked lasers [20], broadband superluminescent diodes and lasers [21], as well as broadly−tunable lasers [7,22].…”

“…Another solution is to use a quasi−Littrow configuration (Figure 3c), where the light is collimated onto the diffraction grating, and the diffracted light is then fed back through the diode, for spectrally selective laser emission. Each of these schemes come with their own advantages: while the Littrow and quasi−Littrow setups are the easiest to implement and enable higher output powers [7] (and are thus the most popular), the Littman−Metcalf cavity is capable of achieving very narrow linewidths [33]. Other schemes might simply incorporate a tunable filter and/or a Fabry−Perot etalon in the cavity instead of a diffraction grating [34].…”

“…Since quantum dots (QDs) were realized in the 1970s and 1980s [1−3] and first implemented in lasers in the 1990s [4,5], they have proven to have many useful qualities, such as low threshold current OPEN ACCESS density [6], high output power [7] and wide wavelength tunability [8]. In the past years, a range of epitaxially−grown QD materials systems has emerged, able to cover the spectral region from 638 nm [9] up to 1.9 µm [10].…”