Feedback insensitive integrated semiconductor laser

External optical feedback can severely deteriorate the performance of semiconductor lasers. This chapter proposes an integrated laser design that can withstand tens of percent of off-chip feedback, without requiring the integration of magneto-optic materials. The proposed laser consists of a ring cavity with a weak intracavity optical isolator. Sufficient gain difference between clockwise and counterclockwise modes leads to unidirectional laser oscillation. Any reflected light is returned to a mode that is below threshold. This significantly reduces interactions between the feedback and the lasing mode. A rate-equation analysis is presented to show that for 10 dB of intracavity isolation the relative intensity noise changes less than a factor 2 when less than −10 dB of the light is fed back into the laser and when intracavity isolation is 10 dB. Linewidth and optical output power change approximately 0.1 % for an external reflectivity up to −0.1 dB. Semiconductor lasers can be particularly sensitive to EOF, as was recognized long ago [6, 60–62]. Five feedback regimes have been identified in a distributed feedback laser based on the feedback power ratio and the distance to the reflection [6]. The distance to the reflection only plays a role for ratios below −50 dB. Going from low to high reflection, feedback can cause external cavity modes, line narrowing, extreme line broadening and finally stabilization when the laser field is dominated by the feedback. In many circumstances the strength of the reflection is unpredictable and can fluctuate over time, resulting in an undesirable change in the output laser light. To solve this problem, one or often two Faraday isolators are employed to prevent 1 This chapter was published as [1], © 2018 IEEE. 16 Chapter 2 Model without intracavity coupling EOF from returning to the laser cavity altogether [63–66]. From [6] it is found that feedback power ratios below −60 dB have no observable effect on the lasing mode of a distributed feedback laser for typical feedback distances. Attempts to obtain Faraday isolation on chip have been made [40, 41, 43, 44, 48, 67–72]. So far the insertion loss and integration complexity have proven a barrier to implementation of non-reciprocal elements however. Recently a class of isolators has been devised which avoids the use of magneto-optic materials altogether. It was found that isolation can be achieved using non-linearities [73], but this type of isolator can only be used as when light is either propagating in the forward or the reverse direction. These devices are therefore not suitable for preventing the detrimental effects of EOF on continuous wave lasers, but can be used for pulsed lasers as was proposed in [74]. Isolation can also be obtained using a time varying refractive index [50, 52, 53, 75], but none of these have achieved the 60 dB required to fully suppress the effects of EOF. This work takes a different approach to suppressing the effects of EOF on a continuous wave, single-frequency, tunable laser. The principle is to allow light to get back into the laser cavity, but into a different, sub-threshold mode. This is achieved by placing a weak isolator inside the cavity of a ring laser such that one direction is favored over the other. E.g. the clockwise (CW) mode reaches threshold and is lasing, while EOF returns to the counterclockwise (CCW) mode. With a gain difference of only 0.5 dB the CCW mode is suppressed by approximately 40 dB relative to the CW mode [76]. For large external reflection, EOF contributes significantly to the power in the CCW mode and larger isolation is required, at most 10 dB as will be shown later. It will be shown that by using a weak intracavity isolator the effects of EOF on the lasing mode can be greatly reduced. This is in stark contrast to a ring laser that is forced to oscillate unidirectionally using an external mirror, which was studied using a rate equation analysis in [77]. As was shown in [52], it is possible to achieve the required isolation using InP based waveguides and phase modulators but the principle holds for any integration platform that is able to create 10 dB of isolation. A less detailed study of the steady state behavior of this laser was already performed by us in [78], while [79] studied the dynamic behavior of the laser in some detail. In this chapter we will analyze an integrated ring laser with intracavity isolator in detail to obtain a good insight into the performance of the laser when it is subjected to EOF and to determine the requirements on the isolator. Section 2.1 first gives an intuitive explanation why a unidirectional ring laser with weak intracavity isolator will be less susceptible to EOF. Section 2.2 then goes on to describe the model that is used to analyze the laser providing the assumptions which have been made. Section 2.3 details the analysis of the laser for the steady state, resulting in the average values of the optical intensity and number of charge carriers, and section 2.4 introduces the effects of spontaneous emission noise. As a result of this analysis the RIN spectrum and linewidth are obtained as a function of the feedback rate and intracavity isolation. These values are used to quantify the performance of the laser. Finally, section 2.5 concludes the chapter. 2.1 Concept of laser operation 17 −−−−−→ isolator coupler