COUPLED CAVITY LASER

Abstract: An internally mode stabilized laser comprising a semiconductor laser cavity (10) optically coupled to a passive optical cavity (12) having a reflective coating (14) on its end remote from the laser cavity. The passive cavity is a high efficiency waveguide (20) with dimensions chosen to suppress unwanted modes.

Description: COUPLED CAVITY LASER Background of the Invention Single frequency laser sources are of great interest for high bandwidth single-mode fiber communication systems. Longitudinal mode selection by a coupled cavity arrangement comprising a laser cavity optical source optically coupled to a second optical cavity has been investigated as an approach to achieve single- longitudinal-mode (single frequency) operation of semiconductor lasers. (See D. A. Kleinman and P. P. Kisliuk, The Bell System Technical Journal, Volume 41, p. 453, March, 1962 and P. . Smith, Proceedings of the IEEE, Volume 60, No. 4, p. 422, April, 1972). In such coupled cavity devices, light from a multi-longitudinal mode laser is coupled to the second cavity and reflected back to the laser cavity. By proper selection of the relative dimensions of the two cavities, a resonant condition is obtained in which a desired single frequency output is obtained. In effect, unwanted longitudinal modes are suppressed in favor of the desired mode. The second cavity optically coupled to the laser can be either a passive cavity comprising a light reflector spaced, in air, from the laser, or a second laser cavity with bias-current control. (D. Renner and J. E. Carroll, Electronics Letters, Volume 15, No. 3, p. 73, February 1979; C. Lin and C. A. Burrus, Post Deadline Paper No. PD5-1, Topical Meeting on Optical Fiber Communication, New Orleans, February 28-March 2, 1983; L. A. Coldren, B. I. Miller, K. Iga, and J. A. Rentschler, Applied Physics Letters, Volume 38, p. 315, March, 1981; and K. J. Ebeling, L. A. Coldren, B. I. Miller, and J. A- Rentschler, Applied Physics Letters, Volume 42, p. 6, January, 1983). A long laser cavity and a shorter second passive, or active, cavity are used in such a coupled cavity geometry. The passive-active coupled cavity arrangement is attractive for its simplicity, because, for example, the second cavity can be readily combined with known and commercially available lasers. In such combination, a concave spherical mirror has been used to reflect the laser light, in a narrow beam to reduce losses, back to the laser. (See K. R. Preston, K. C. Woollard, and K. H. Cameron, Electronics Letters, Volume 17, No. 24, p. 931, November, 1981). However, manufacturing and alignment of the spherical mirror is difficult. A coupled planar Au mirror can be used to provide a short passive cavity. A longer passive cavity cannot be provided with a flat mirror because of the poor optical coupling efficiency between the two cavities caused by light beam dispersion. Statement of the Invention An optimized coupling efficiency between the active laser and the passive cavity is required for reliable single-mode operation. This is achieved according to the invention by coupling to the laser a passive graded index lens stabilizer cavity. The passive cavity is terminated with a highly reflective surface. The passive cavity may be attached to or be in close proximity to the laser. The passive cavity has an optical length that is less than the corresponding optical length of the active laser cavity. This insures stable suppression of unwanted longitudinal modes. Preferably, the coupled graded index cavity has a length slightly less than a quarter pitch of the light path such that the reflected light is focused at the front face of the laser. Description of the Drawing FIG. 1 is a schematic representation of the passive to-active coupled cavity laser of the invention; FIG. 2 is a diagram of a first embodiment of the invention; FIG. 3 is a diagram of another embodiment of the invention; FIG. 4 is a diagram showing continuous wave and square wave pulsed spectra for the laser alone (a) and for the coupled cavity arrangement of the invention (b) . Detailed Description The basic arrangement of the inventive coupled cavity laser is shown in FIG. 1. An active laser cavity 10 is shown in close proximity to a passive graded index (GRIN) cavity 12. An air space of 10 μ is shown between the active and passive cavities. Alternatively, the two cavities may be butted, or even joined together with epoxy or the like. If separated as shown it is preferred that they be close, e.g. within 50 μ, and preferably within 10 μ, to maximize coupling. The front end of the graded index cavity facing the laser can be antireflection coated in case reflection from that surface affects mode stability. The passive graded index cavity is bounded (on its left, as viewed in FIG. 1) by a totally or partially reflecting mirror 14. The reflectivity of the mirror depends on the amount, if any, of radiation desired from that end of the coupled cavity assembly. The coupled cavity device can be designed for output coupled to a transmission fiber 16 through the mirror as shown in FIG. 2, in which case the mirror is designed to pass the desired amount of light. Also, the device can be designed with a power (or even a spectral) monitor 18 at the other facet of the active cavity as shown. FIG. 3 shows an alternative arrangement with the output radiation taken from the active cavity of the device. The monitor is optically coupled to the partially transmission mirror. In each instance, the axes of the two cavities are in alignment. Further details of the device are as follows. GRIN cavities with a short length compared to the laser cavity length were prepared from graded index multimode fiber. The fiber 20 was bonded inside a glass capillary tube 22 (FIG. 2) , and was polished from both ends to a predetermined length. One end was then coated with a layer 14 comprising Cr and Au for high reflectivi y. The coated end was then bonded to another capillary tube 24 having a multimode fiber 26 disposed therewithin. This latter tube-fiber member 24-26 does not affect the operation of the coupled cavity laser which "ends" at the CrAu layer. The member 24-26 serves, however, as a convenient means for handling the GRIN cavity and for coupling it to a conventional light fiber coupler 28. The member 24-26 can be omitted. The mode spacing of an InGaAsP BH laser 10 is 1% , from which the effective cavity length in air is 1200μm. The air gap between the GRIN cavity 12 and the laser facet 30 was «6l0μm. This air spaced cavity did not contribute to the mode selection effect due to its large cavity mode spacing (r 120x laser mode spacing) . The graded index fiber 20 used for the GRIN cavity had a 0.29 numerical aperture and 70μm core diameter. The length of the GRIN cavity 12 was 170μm which was made shorter than a quarter pitch to have a focal point in air. This results in nearly a 4:1 ratio of the external cavity mode spacing to the laser mode spacing. A ratio chosen between 3:1 and 6:1 would be appropriate. By employing the coupled cavity approach described here it is also appropriate to use shorter active laser cavities than one might otherwise find expedient from the standpoint of laser lifetime. The 6m /facet continuous wave spectrum for the InGaAsP BH laser biased at 51mA (I th =23mA) at 23 degrees C is shown in FIG. 4a. A maximum of 40% increase in light output was observed when the GRIN cavity was coupled to the laser. The corresponding 6mW continuous wave single- longitudinal-mode spectrum at the same heat sink temperature is shown in FIG. 4b. The laser with the coupled external cavity was pulsed by a square wave generator from threshold to 6mW. A beam splitter was used for simultaneous measurements of the spectrum and the light output pulses detected with a high speed InGaAs/InP PIN photodiode. (See T. P. Lee, C. A. Burrus, K. Ogawa, and A. G. Dentai, Electronics Letters, Volume 17, No. 12, p. 431, 1981). The on/off extinction ratio was better than 30:1. The spectra of the GRIN external cavity laser under 300MHz and 500MHz square wave modulation are also shown in FIG. 4b. The corresponding multimoded spectra for the laser alone are shown in FIG. 4a. The same longitudinal mode was stabilized with the external cavity as the modulation rate was varied. The most significant side mode intensity relative to the main mode intensity was suppressed by 23dB and 20dB respectively at 300MHz and 500MHz. Under continuous wave operation the side mode suppression ratio was better than 25dB. When the laser was pulsed from above Ith' ^he amplitude of the first relaxation oscillation peak decreased and the mode suppression ratio approached that under continuous wave operation with the penalty of a decreased on/off light output extinction ratio. i -OMPI

Claims: Claims 1. A cavity coupled laser device comprising a semiconductor laser cavity (10) and a passive optical cavity (12) coupled thereto for influencing the output mod of the laser cavity, characterized in that said passive optical cavity comprises a graded index lens (12) with its axis aligned to the output axis of the laser cavity, and a reflecting coating (14) on a surface of the graded index lens so that light emerging from the laser cavity traverse an optical path through the graded index lens and onto sai reflective coating, the length of the optical path through the passive cavity being less than the length of the optical path in the laser cavity. 2. The device of claim 1 in which the graded index lens is a length of glass fiber waveguide (20). 3. The device of claim 1 in which the length of the optical path one way through the passive cavity is slightly less than 1/4 pitch.

Inventors: LIOU KANG, LIOU, KANG
Applicants: AMERICAN TELEPHONE & TELEGRAPH, AMERICAN TELEPHONE & TELEGRAPH COMPANY
Application year: 1984
Main classification: H01S3
IPC codes: G02B6, H01S3, H01S5