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A Compact Tunable Narrow-Band LD Based on Emission Through the Substrate and an External Abnormal-Reflection Mirror N. V. Baidus, I. F. Salakhutdinov, H. J. W. M. Hoekstra, B. N. Zvonkov, S. M. Nekorkin, and V. A. Sychugov

Abstract—Due to the high beam divergence of standard laser diodes (LDs), these are not suitable for wavelength-selective feedback without extra optical elements. This letter reports on first prototypes of compact tunable LDs with wavelength-selective feedback via a so-called abnormal reflecting mirror of a low divergent beam coming from a special LD, emitting through the substrate. With this setup, tunable narrow-band ( 0.7 nm) lasing is observed showing good temperature stability. Tuning over a range of 5 nm at a maximum power of 0.2 W was demonstrated. Index Terms—Gratings, laser mirrors, laser tuning, stability.

I. INTRODUCTION

S

EMICONDUCTOR laser diodes (LDs) are very valuable compact high-power sources of coherent radiation [1]. However, high-power LDs usually show unwanted effects like multimode radiation with rather broad spectra. From this point of view, it is interesting to look for new approaches to improve the performance of LDs. One of the possibilities for such improvements is to use external mirrors. An element that seems quite suitable to do so is the so-called abnormal reflection mirror (ARM), i.e., a waveguide-grating structure based on the effect of abnormal light reflection [2]–[5]. These structures have the property of ideally 100% zeroth-order reflection at a given wavelength and polarization, for an incoming plane wave at an angle of incidence corresponding to excitation of the leaky mode in the structure. For monochromatic incoming beams with an angular width of 1–2 , the reflectance may be as large as 70%–90%. These structures can be used as a narrow-band reflection filter [6], semiconductor waveguide grating [7], as well as for excitation of long-range surface plasmons in corrugated metal films [8]. It was demonstrated [9], [11] that using such ARMs in an extended cavity, also including lenses, with an LD may lead to narrow-band lasing (down to 0.1 nm) and tunability up to a Manuscript received May 22, 2001; revised August 2, 2001. This work was supported by Netherlands Organization for Scientific Research (NWO) under Project 047.006.014. N. V. Baidus is with the Institute of Physical and Technological Research, 603950 Nizhni-Novgorod, Russia, and also with the Department of Applied Physics, MESA+Research Institute, U-Twente, 7500 AE, Enschede, The Netherlands (e-mail: [email protected]). I. F. Salakhutdinov and V. A. Sychugov are with the General Physics Institute, 119991 Moscow, Russia. H. J. W. M. Hoekstra is with the Department of Applied Physics, MESA+Research Institute, University Twente, Enschede, The Netherlands. B. N. Zvonkov and S. M. Nekorkin are with the Institute of Physical and Technological Research, 603950 Nizhni-Novgorod, Russia. Publisher Item Identifier S 1041-1135(01)09207-2.

Fig. 1. Scheme of the setup for the LD with external ARM. Tuning of the laser wavelength is possible by rotation of the ARM around the axis indicated at the left-hand side. The numbers indicate: 1. Active WG core, 2. HR coated end face, 3. AR coated front face, 4. WG beam and 5. Dominant substrate beam.

range of 18 nm. This letter is a continuation of previous work with LDs emitting through the substrate (LDES). An important property of the LDES is that the emitted substrate beam has a low divergence (typically 1.5 ) [10]. In earlier experiments [11], we used an extended resonator consisting of an LDES, focusing lens, ARM, and spherical mirror. The narrow beam of the LDES was focused on ARM, at the angle corresponding to maximum reflection. The reflected ray was directed to a spherical mirror reflecting the light back via ARM and lens to the LD. It was found that the full-width at half-maximum (FWHM) of laser radiation spectra was as low as 0.3 nm with virtually no temperature dependence. Rotation of the ARM showed that the laser was tunable in a range of 10 nm. Based on the above experiments, we have developed a compact lasing setup with comparable features, but with the ARM placed directly in front of an LDES, supplied with an AR ( ) coating of the front facet. The ARM introduces wavelengthselective feedback of the substrate beam. II. PRINCIPLE OF THE DEVICE The studied laser device consists of only InGaAs–GaAs–InGaP quantum-well (QW) LD [10] with ) emitting through high-transparency front facet ( the substrate (Fig. 1) in the 990-nm wavelength region, and an ARM in front of it. The latter was placed such that the angle of full reflection for TE-polarized (electric field was equal to the perpendicular to plane of Fig. 1) light . We will angle of the radiation leaving the LD substrate first discuss the LDES without the ARM in front of it. The structure radiates three beams. The first beam (as usual for a

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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 13, NO. 11, NOVEMBER 2001

Fig. 2. Angular far-field scan of the output of a LDES without ARM. The two peaks correspond to the two substrate beams.

semiconductor laser) is due to the (leaky) waveguide (WG) modes and leaves the LDES in a direction perpendicular to the front facet. The beam divergence in the plane perpendicular to p–n junction ( – plane) is about 50 (see Fig. 2). Due to tunnelling of the forward and backward running WG mode, there are two substrate beams of which only one is dominant. This can be understood as follows: the (leaky) WG modes , running from left to right has an intensity where is the amplification coefficient including loss by is the distance from the front facet. The tunneling, and mode running to the left has more intensity due to longer , where is the length of path length the active region. The divergence of the substrate beams is 1.5 , corresponding to the output aperture of 180 m, allowing for high feedback (due to low angular width) via an ARM structure in front of the structure. The angular width in the other transverse direction is for all three beams, due to multimodality of the WG modes 10 –12 . The outgoing substrate beams leave the front facet at an angle of with the normal. The ARM with high reflection coefficient (reflectance 60% for an angular width of an incoming beam of 15 ) was made by deposition of Si N layer on a glass substrate. The layer thickness was 278 nm, etching depth 100 nm, corrugation period nm, leading to an amplitude coupling length of 20 m. The abnormal reflection angle for 999-nm wavelength radiation is equal to 30 , if the grooves of the grating are perpendicular to the plane of incidence ( – plane). The ARM should be placed close to the end-face of the LD in order to have good reflection back into the substrate. For this reason, wavelength tuning by rotating around an axis parallel to the grating grooves is no longer possible as this would damage the LD end face. Instead, we have used rotation around the axis for tuning (see Figs. 1 and 3). The condition for optimum reflection, corresponding to excitation of the leaky WG-grating mode is given by (1) where modal propagation constant; projection of the wavevector of the incident wave on the plane of the grating ( – plane in Fig. 3); the grating wavevector.

Fig. 3. Schematic picture of ARM by changing a different wavelength may be selected for given  .

Fig. 4. Spectrum of LDES-ARM at = 90 .

By solving this equation for the wavelength of the excited mode, we obtain (2) Here, angle of radiation incidence on the grating; effective refractive index of the excited waveguide mode; corrugation period of the grating. Equation (2) relates the wavelength of the excited mode to the angle of incidence and the angle between the grating grooves and the plane of incidence. Note that according to (2), corresponds to the maximum wavelength for high reflection. III. RESULTS AND DISCUSSION With an ARM in front of LDES, emitting in the 960–1000 nm region we found, as anticipated, only one dominant substrate beam and the WG beam leaving the setup (i.e., beams 4 and 5 according to Fig. 1). On the latter, we have performed experiments. It was observed that the angular width of the WG beam in a direction parallel to the p–n junction was decreased (from 10 –12 ) to 1 –2 , indicating that ARM introduces a feedback preferentially for the lower order WG modes. Narrow-band lasing was observed (see Fig. 4) for an arrangement lasing . Tuning of at a wavelength of 999 nm for an angle

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has been investigated. The obtained results have shown that this approach can improve characteristics of LD from the point of view of tunability, beam divergence, and thermal stability. The properties of laser radiation are compatible with properties of -DFB lasers [13]. The stable single-mode generation opens the possibility of using the field enhancement in a nonlinear ARM for a direct frequency doubling of radiation [12].

REFERENCES Fig. 5.

Spectra of LDES-ARM at different angles .

this sample was not possible as there was only a small overlap between the wavelength region of highest gain (999–1004 nm), and the region of high reflectivity of the ARM (below 999 nm) for given angle of incidence. Tunability was observed for other LDES having its region of maximum gain at 965 nm, over a wavelength of 5 nm (see Fig. 5). The side bands are attributed to the relatively low quality of the AR-coating at the end face. The above experiments have shown the presented approach to be quite promising. It seems worth while to also investigate the potential of using the dominant substrate beam (instead of the WG beam) as main output. In both cases, the performance of the device with respect to output power, spectral properties and tunability can be improved by increasing the strength of the grating and the high transmission coating on the front facet of the LDES, and optimizing the coupling strength between modes in the active waveguide and the substrate and the length of the LDES. Besides the positive features of the presented setup mentioned above, it is noteworthy that the scheme is also quite promising for nonlinear effects, such as second-harmonic generation (SHG). If a nonlinear substrate like KTP is used for the fabrication of the ARM and the structure is optimized for efficient SHG, the field enhancement related to the excitation of the (leaky) mode in the ARM may be used for efficient SHG [12]. IV. CONCLUSION The possibility of using an ARM as an external mirror in the cavity of a semiconductor LD emitting through the substrate

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