Abstract First phase A steptunable external cavity laser

Abstract First phase A step-tunable external cavity laser with two Fabry-Pérot etalon filters is demonstrated. The angle of one etalon induces a step-tuning by 100 GHz. And the possibility that a steptuning is induced by the variation of a refractive index is shown. Second phase I propose a new external cavity laser which can be step-tuned based on the Vernier effect between a Fabry-Pérot etalon and the longitudinal mode of an external cavity.

Widely Tunable External Cavity Lasers Based on the Vernier Effect M. Kinoshita Outline 1. Introduction 2. Principle External cavity laser Vernier effect 3. Experiments 4. Summary

Introduction optical transmission networks Wavelength Division Multiplexing (WDM) which let us have large transmission capacities is very important system for the next generation communication. This system needs widely tunable lasers in order to become more efficient. semiconductor laser l 1 semiconductor laser l 2 semiconductor laser … at present … Fixed wavelength lasers are used on the each channels. l 3 semiconductor laser ln multiplexer ～ ～ The space of each channels is usually 100 GHz (=0. 8 nm).

Purpose The realization of the step-tuning of the semiconductor laser’s frequency spectral image 100 GHz Intensity (a. u. ) 100 GHz ・・・ 0 detuning (GHz) 100 detuning (GHz) 200 detuning (GHz) 300 detuning (GHz) We expect that laser frequency is step-tuned by 100 GHz.

Sampled Grating DBR laser based on Vernier effect Sampled Grating 1 Phase Gain Sampled Grating 2 R 1 Beat This monolithic array is complicated and requires a high processing technique, although it is compact. In this study We use the external cavity lasers because of its simplicity, expandability, and thermal stability.

External Cavity Diode Laser Usually … In the case of external cavity lasers … AR coating Both side facets act as Fabry-Pérot resonator. The facets have AR-coating. And we made a resonator outside. Depending on the form of the external cavity, various tuning can be achieved. For example, single mode tuning, continuous tuning, and widely step-tuning can be done.

External Ring-Cavity Laser mirror PBS isolator Laser Diode Specification cavity length feedback ratio output power linewidth 100 mm 386 mm 60 % 1. 7 m. W (at 70 m. A) 50 k. Hz

Fabry-Pérot etalon transmittance the velocity of light c L reflectance R loss A frequency n Free Spectral Range finesse transmittance refractive index n 1 0. 5 0 transmittance FSR FWHM frequency n

Vernier effect transmittance Two etalons have slightly different FSR each other 1 individual 0 0 transmittance 1 individual frequency transmittance 0 1 beat transmittance frequency 0 frequency transmittance revolve one etalon transmittance 1 beat transmittance

Transmission spectra of the etalon filters FSR=95 GHz, finesse=5. 1 0. 1 transmittance 1 195. 8 196. 2 196. 4 frequency (THz) 0 196. 6 195. 8 196. 2 196. 4 frequency (THz) 196. 6 beat 0. 1 transmittance 0 FSR=100 GHz, finesse=36 0 resolution： 6. 4 GHz 195. 8 196. 2 196. 4 frequency (THz) 196. 6

The first phase Step-tuning using two etalon filters etalons angle 6~6. 5 deg 0 deg FSR 95. 0 GHz 100 GHz Finesse 5. 1 36 polarizing beam splitter (PBS) l/2 plate optical isolator mirror lens LD laser diode spectrum analyzer

Experimental result step-tuning by the angle of the etalon filter 16 ch intensity (a. u. ) 100 GHz q (deg) 6. 1 6. 2 6. 3 6. 4 195. 5 196. 5 frequency (THz) 197 6. 5 197. 5

Analysis We calculate the dependence of the laser frequency (= the peak of two etalon’s beat) on the etalon’s angle. The calculated beat spectrum transmittance 1 100 GHz 0. 5 Dq 0 Frequency transmittance 1 100 GHz 0. 5 0 Frequency

The dependence of laser frequency on the etalon’s angle laser frequency (THz) 197. 5 197 196. 5 experiment calculation 196 195. 5 195 5. 9 6. 0 6. 1 6. 2 6. 3 angle of etalon (deg) 6. 4 6. 5

Problem The loss of the etalon filters increases threshold current and reduces the maximum output power. without etalons 2. 0 0. 2 0. 15 power (m. W) 1. 5 power (m. W) with two etalons 1. 0 0. 1 0. 05 0. 5 threshold 0 0 10 20 30 40 50 60 current (m. A) 70 80 90

We tried to induce the step-tuning by the variation of a refractive index n, not the angle of the etalon. So far refractive index slow We used the mechanical control which has a slow reaction velocity. Next fast We are going to use the electrical control which has a fast reaction velocity.

1. 3 or 1. 46 mm wavelength semiconductor laser chips are used as the etalon filters with the variability of a refractive index. Because … We would expect that the peak of the transmission can be shifted of dozens GHz by the carrier plasma effect. Both side facets act as Fabry-Pérot resonator from the beginning. V

Laser chips 1. 46 mm laser chips 1. 3 mm laser chips 300 mm 100 mm

Variation of the longitudinal mode by the injected current Transmission of the 1. 46 mm laser chip 194. 35 Variation of the peak frequency 194. 2 frequency (THz) transmission (a. u. ) current 194. 3 194. 25 194. 3 frequency (THz) 194. 35 194. 2 0 injected current　(m. A) 1

Problem transmittance 1 25 GHz 0 1 transmittance individual transmittance frequency beat transmittance between two etalons 25 GHz 0 longitudinal mode frequency We should consider the longitudinal mode of an external cavity as well as beat of two etalons. transmittance 1 0 1 GHz frequency

The second phase Vernier effect between a Fabry-Pérot etalon and the longitudinal mode of an external cavity external mirror lens etalon Phase AR HR × frequency beat etalon’s mode transmittance cavity’s mode transmittance Gain frequency

Simulation of the lasing spectra using the transfer matrix Transfer matrix L Er+ ＝ t. Ef+‐r. Er- Ef+ t r Ef+ Er- Ef- = r. Ef+ + t. E- Er+ ＝ t. Ef+exp(-ik. L) M M Transfer matrix of a reflector Er. Ef- = Er-exp(-ik. L) P P Transfer matrix of space

Result of the simulation The calculated lasing spectrum intensity (a. u. ) SMSR > 30 d. B frequency (Hz) Lasing frequency is shifted to the next channel by variation of the refractive index about 10 -4.

Summary We have realized the external cavity laser with two etalon filters tuned in step of 100 GHz from 1522. 8 nm to 1534. 5 nm. The longitudinal mode of the 1. 46 mm wavelength laser chip was shifted over its FSR by the injected current’s variation of 1 m. A. It suggest that the step-tuning induced by the variation of the refractive index can be achieved. The spectrum of the step-tunable laser based on the Vernier effect between a Fabry-Pérot etalon and the longitudinal mode of an external cavity was simulated.