Superfluorescent source

Abstract: A superfluorescent broadband fiber laser source comprises an optical fiber 10 doped with a laser material and having one end pumped with pump light of sufficient intensity to produce amplified spontaneous emission within the fiber and forward and backward waves therefrom. The polarisation of the backward wave experiences a 45° rotation as it travels into and another 45° rotation as it passes back out of a 45° Faraday rotator mirror device 26. An amplified polarisation-rotated beam from the doped fiber is divided into orthogonally polarised beams in a polarisation beam splitter coupler 32. A return polarised signal from an interferometer 40 is passed through the doped fiber 10, the Faraday rotator mirror device 26 and again through the doped fiber 10 for producing a polarised output signal that has been amplified by four passes through the doped fiber.

Description: Superfluorescent broadband doped fibre laser sources are described in U.S. patents numbers: 4,637,025 to Snitzer, et al; 4,938,556 to Digonnet, et al, which is a continuation in part of 4,859,016 to Shaw, et al, which is a division of 4,723,824 which is a division of 4,674,830 (all for fibre optic amplifiers); and 5,136,600 to Fidric, et al. The superfluorescent fibre lasers and amplifiers in the patents generally comprise an optical fibre having it's core or cladding or both doped with an active laser material and pumped at an appropriate frequency and sufficient intensity to produce amplified spontaneous emission (ASE). These patents disclose both single pass and double pass configurations. In a single pass configuration the output signal results from only one pass amplification of the spontaneous emission. In a double pass configuration a dichroic filter or a mirror and coupler are employed to reflect the backward spontaneous emission signal (not the pump signal) back through the doped fibre for second-pass amplification of the spontaneous emission. Unfortunately these superfluorescent sources are quite sensitive to optical feedback of the output signal in the doped fibre, which causes an onset of oscillating modes and a resultant narrowband emission. Such an oscillating condition prevents operating these devices as high power broadband light sources. An alternate approach is a resonant laser light source described in U.S. patent number 4,964,131 to Liu, et al. which is pumped at a wavelength selected to be outside the pump variable tuning range of the lasing material, e.g., Neodynium. This resonant laser has been found to be less efficient and considerably less stable wavelength wise in comparison to the referenced double pass fibre sources. Electronics Letters, vol. 28, no. 12, 4 June 1992, pages 1126-1128 discloses an apparatus according to the preamble of claim 1. According to the present invention there is provided a superfluorescent source apparatus for providing optical power, the apparatus comprising an optical fibre doped with laser material, a pump source for providing pump light to one end of the optical fibre to induce in the optical fibre superfluorescent light emission at wavelengths centred upon an emission wavelength, and means for returning light received from the other end of the optical fibre back to and through the optical fibre and then to an input/output coupling means, means for rotating the polarisation of light which has exited from the fibre at said other end so that the polarisation of light exiting that other end is different from the polarisation of the light returning to that other end, and polarisation sensitive means having three ports of which a first port is coupled to said one end of the optical fibre for receiving light which has exited from the optical fibre, a second, output, port and a third port connected to said input/output means, the polarisation sensitive means being operable to pass from the first to the third port substantially only light of a given polarisation and vice versa, and to couple substantially only light of another polarisation between the first and second ports and vice versa, characterised in that the apparatus is in combination with a utilisation device, which device has input/output means for receiving light and for delivering an output light signal produced by the utilisation device from the light received the input/output means being connected by said input/output coupling means to said third port whereby the utilisation device will receive light which has traversed the optical fibre twice, whilst its output light signal will also traverse the optical fibre twice to provide signal amplification prior to exiting by way of said second port. In an amplifier, the pump signal and a signal to be amplified are both launched into the one end of the doped fibre. In a preferred embodiment the rotating means comprises a Faraday rotator mirror device which rotates the polarizations of the backward wave a total of 90° as it travels into and back out of the Faraday device. The polarization-rotated amplified forward wave emission signal from the one end of the fibre is applied to the poarisation sensitive means in the form of a polarization beam splitter coupler for providing orthogonally polarized output signals at the second and third ports thereof. The pump signal and polarization rotated amplified forward wave signal may be coupled to and from the one end of the doped fibre by a wavelength division multiplexer coupler. Embodiments of the invention may be designed to provide a source that has an efficiency and a stability comparable to conventional double pass configurations and which has as high a resistance to optical feedback as the single pass approach. Embodiments may also provide a source of polarization selective superfluorescent emission output light for a utilization device such as an interferometric sensor, which light is reapplied to the doped fibre for producing an amplified output signal having a prescribed polarization; that is the result of photons being amplified four times through the gain medium of the doped fibre. In a preferred embodiment, the interferometer sensor is used as a gyro for sensing rotation of the sensor. In use, processing circuitry will accordingly be coupled to the second output port. For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying Figure 1 which is a schematic block diagram of one embodiment of this invention. An interferometer system embodying this invention is shown in Figure 1 where the system comprises a length of erbium doped fibre 10, for example, having one end 12 connected through a length of fibre 16 to a wavelength division multiplexer coupler 18 and having its other end 14 connected through a length of fibre 24 to a Faraday rotator mirror device 26; a source 22 of pump light signal; a polarization beam splitter coupler 32; and a Sagnac interferometer 40 comprising a multifunction integrated optic chip (MIOC) 42 (comprising a polarizer 44, a Y-junction 46, an input lead 48 and two output leads 50 and 52) and a coil 60 of optical fibre. The couplers 18 and 32 are conventional and both are preferably 4 port devices that are connected together and to the doped fibre 10 and to the interferometer 40. The x's in the drawing indicate places where various fibres are spliced together. The fibres 30, 34, 36 and 48 are preferably polarization maintaining fibres (PMF). The birefringence axes of fibres 36 and 48 should be properly aligned during the splicing operation. In contrast, the fibres 10, 16, 20, 24 and 28 may be regular single mode fibre (SMF), except for proper doping of gain fibre 10. Wavelength division multiplexer couplers are frequency sensitive devices that are described in U.S. patents numbers: 4,556,279; 4,515,431; and 4,674,830, which are incorporated herein by reference. The multiplexer coupler 18 is operative for coupling substantially 100% of the pump signal wavelength into port 1 and out of port 2 for being launched into the one end 12 of the doped fibre 10. This coupler 18 is also operative for coupling substantially 100% of the signal wavelength emission in the forward direction (indicated by arrow 11) from the doped fibre into multiplexer coupler port 2 and out of port 3 for transmission to port 1 of the coupler 32. The pump signal is blocked from this output port 3 of the multiplexer coupler. Port 4 of the multiplexer coupler is coupled to a light absorbing termination T. Polarization beam splitter couplers are polarization sensitive devices and are described in ones of the U.S. patents: 5,150,370; 5,127,066; 5,030,842; 4,985,898; 4,886,334; 4,063,084; and 3,976,838; and are commercially available devices (e.g., model AFOC-S-1.55, available from Photonetics). The splitter coupler 32 is operative for coupling 100% of only one polarization between ports 1 and 2 and onto the output fibre 34, and for coupling 100% of only another orthogonal polarization between ports 1 and 4 and to a utilization device such as the interferometer 40. The port 3 of splitter coupler 32 is connected to a light absorbing termination T. The Faraday rotator mirror device 26 is a commercially available device and may be a model I-15-FRM manufactured by ISOWAVE. This Faraday rotator mirror essentially comprises a thin plate of magneto-optic material having a magnetic field applied thereto in the direction 15 of the backward emission (i.e., orthogonal to the plane of the face of the plate), with the backside of the plate being mirrored. The thickness of the plate is selected to provide 45° polarization rotation to a light beam passing through it to the mirrored surface and another 45° polarization rotation as this light beam travels back through the magneto-optic material. Thus, the Faraday rotator mirror device 26 not only reflects the backward wave superfluorescent emission, but also rotates its state of polarization by 90°. A commercially available Faraday rotator mirror device usually has a fibre pigtail and a microlens associated with it to facilitate coupling the light to the magneto-optic plate. The interferometer 40 is conventional and comprises a multifunction integrated optic circuit (MIOC) 42 and a coil 60 of preferably polarization maintaining fibre. The circuit 42 comprises a polarizer 44 having its axis aligned with one of the birefringence axes of the input fibre lead 48 and a Y-junction (46) that divides the input signal from the splitter coupler 32 and combines the output signals from opposite ends 54 and 56 of the coil 60. The circuit 42 is connected to port 4 of the polarization beam splitter coupler (PBSC) by a length of polarization maintaining fibre 36. Although a Sagnac interferometer is illustrated in the drawing, typically most any interferometer may be used here. Also, other types of utilization devices such as an optical time domain reflectometer may be connected to the output port 4 of the splitter coupler. The doped optical gain fibre 10 is conventional. The doped fibre 10 is a superfluorescent source (SFS) of amplified spontaneous emission at an optical frequency when it is pumped with light of appropriate (usually shorter) wavelength and of sufficient intensity. A theoretical analysis of superfluorescence or amplified spontaneous emission is found in U.S. patent number 4,938,556 to Digonnet, et al. at columns 9-11 and the article Theory of Superfluorescent Fibre Laser by M. Digonnet, et al., Journal of Lightwave Technology, Vol. 4, No. 11, November 1986, pp. 1631-1639. The doped fibre 10 is preferably a single mode optical fibre formed of silica glass and having its core or cladding or both doped with at least one active laser material such as Neodymium or Erbium. The doped fibre lases to produce forward (11) and backward (15) emission (exiting the left and right sides 12 and 14 of fibre 10, as viewed in the drawing) light beams at emission or signal wavelengths in response to pump light of sufficient intensity and appropriate wavelength. In operation, unpolarized pump light from source 22 is coupled between ports 1 and 2 of the multiplexer coupler and launched into the core of the doped fibre. The intensity and wavelength of the pump light are selected to be sufficient to cause the doped fibre to produce amplified spontaneous emission, which is unpolarized light, that is outputted in the forward and backward waves 11 and 15. The forward wave travels between ports 2 and 3 of the multiplexer coupler to be a weak output signal on fibre 28. Indeed, this output signal has been amplified only once as it made one pass through the doped fibre. On the other hand, the backward wave signal 15 also undergoes amplification during a single pass of the doped fibre prior to it's polarization being rotated 90° as it traverses the Faraday rotator mirror device. The polarization rotated reflected backward wave signal is again amplified as it travels from right to left in the drawing through the length of the gain medium of the doped fibre. This causes an increase in the intensity of the resultant emission and a strong output signal on fibres 16 and 28. Since the polarization of the reflected backward wave signal are rotated 90° relative to the incident beam (the initial backward wave 15 in the doped fibre), however, this prevents regenerative amplification of optical feedback from the other end 14 (right) of the doped fibre so as to prevent the onset of oscillations within the doped fibre with the resultant linewidth narrowing of the output signal thereof. Thus, this double pass structure provides a broadband output signal on fibres 16 and 28 that has the intensity of a double pass system and increased stability. Since this light signal on fibre 28 is unpolarized (i.e., it is comprised of orthogonal polarizations that vary randomly), the splitter coupler causes one polarization (e.g., the x-polarization, for simplicity of illustration) to be passed by port 2 onto output fibre 34 and the orthogonal polarization (the y-polarization) to be passed by port 4 to the polarization maintaining fibre 36, polarizer 44 and Y-junction 46 where it is split into equal amplitude beams that are applied as counter-rotating beams of only the y-polarization in fibre coil 60. Any rotation of the interferometer is sensed as a change in the optical phase of the signals in the coil 60 and does not change the polarizations of these signals. The counter rotating beams in the coil 60 are recombined in the Y-junction 46 and passed by the polarizer 44 and polarization maintaining fibre 36 with the same polarization that the light signal had when it was travelling in the opposite direction. This return signal is coupled between ports 4 and 1 of the polarization splitter coupler and ports 3 and 2 of the multiplexer coupler and launched into the doped fibre 10 in essentially the same polarization it had when it left the doped fibre. Although fibres 10, 16 and 28 are not polarization maintaining fibres, by principle of reciprocity, and thanks to the very large speed of light, any polarization effect experienced through these fibres by the initial signal leaving the doped fibre cancels out on its return pass. This return signal is amplified as it passes (in the direction 15) a third time through the doped fibre 10. The polarization of the amplified return signal is then rotated 90° by the operation of the Faraday rotator mirror device 26 prior to its being again amplified as it passes a fourth time through the doped fibre in the forward direction 11. Since the amplified interferometric signal in the forward direction 11 on fibres 16 and 28 has traversed the gain medium of the doped fibre 4 times, it has considerably more amplitude than the corresponding output signal of the prior art double pass system. The second rotation of the polarization of the interferometric signal by the Faraday rotator mirror device, however, again prevents the onset of unwanted regenerative amplification of optical feedback and the resultant onset of oscillations in the gain medium of the doped fibre. The amplified interferometric signal is coupled through ports 2 and 3 of the multiplexer coupler and ports 1 and 2 of the splitter coupler (this signal is x-polarized at this time/point) to the output fibre 34 where it is applied to processing circuitry (not shown). This embodiment has the distinct advantage of operating as a source of a broadband light signal on fibres 16 and 28 that has undergone double pass amplification, but which has increased resistance to oscillation. This embodiment also operates to provide a very efficient broadband fibre source of single polarization light signal at the output ports 2 and 4 of the splitter coupler. This embodiment has the further advantage of providing an output signal on fibre 34 from a utilization device such as an interferometer that undergoes further double pass amplification with the same increased resistance to oscillation. In a conventional double pass system the output signal of the interferometer is typically 10-20 microwatts. The corresponding output signal in fibre 34 should be a number of milliwatts. Although this invention is described in relation to preferred embodiments thereof, variations and modifications will appear to those skilled in the art. By way of example, the light source structure may be used in conjunction with other types of interferometric devices and with utilization devices other than interferometers, and may be operated as a light signal amplifier. Also, the utilization device connected to port 4 of the splitter coupler may be a time domain reflectometer. The reflectometer uses incident light output from port 4 of the splitter coupler to probe an optical circuit, looking for defects therein such as in a splice that will emit back scattered light. The back scattered light will undergo double pass amplification in the doped fibre 10 prior to being outputted on fibre 34.

Claims: A superfluorescent source apparatus for providing optical power, the apparatus comprising an optical fibre (10) doped with laser material, a pump source (22) for providing pump light to one end (12) of the optical fibre to induce in the optical fibre superfluorescent light emission at wavelengths centred upon an emission wavelength, means (26) for returning light received from the other end (14) of the optical fibre back to and through the optical fibre (10) and then to an input/output coupling means (36), means (26) for rotating the polarisation of light which has exited from the fibre at said other end (14) so that the polarisation of light exiting that other end is different from the polarisation of the light returning to that other end, and polarisation sensitive means (32) having three ports of which a first port (1) is coupled to said one end (12) of the optical fibre (10) for receiving light which has exited from the optical fibre, a second, output, port (2) and a third port (4) connected to said input/output means (36), the polarisation sensitive means (32) being operable to pass from the first (1) to the third (4) port substantially only light of a given polarisation and vice versa, and to couple substantially only light of another polarisation between the first (1) and second (2) ports and vice versa, characterised in that the apparatus is in combination with a utilisation device (40), which device has input/output means (48) for receiving light and for delivering an output light signal produced by the utilisation device from the light received, the input/output means (48) being connected by said input/output coupling means (36) to said third port (4) whereby the utilisation device (40) will receive light which has traversed the optical fibre twice, whilst its output light signal will also traverse the optical fibre twice to provide signal amplification prior to exiting by way of said second port (2). An apparatus according to claim 1, wherein light is arranged to pass through the means (26) for rotating the polarisation both after exiting the fibre and again before re-entering the fibre, the polarisation rotation effect being cumulative. An apparatus according to claim 2, wherein the means (26) for rotating and directing comprises Faraday rotator mirror means. An apparatus according to claim 3, wherein the Faraday rotator mirror means is arranged to rotate the polarisation of light by a total of 90°. An apparatus according to any one of the preceding claims comprising means (18) for applying a combination of pump light from the pump source (22) and a light beam from the polarisation sensitive means (32) to the optical fibre (10). Apparatus according to claim 5 wherein said applying means (18) comprises a wavelength division multiplexer (WDM) coupler (18) having at least three ports, said multiplexer coupler coupling only light centered substantially at the pump light wavelength between first and second ports and vice versa, and coupling only light at wavelengths centered substantially at the emission wavelength between the second and a third port and vice versa, the second port being coupled to said one end of the optical fibre, the first port being coupled to the pump source, and the third port being coupled to the first port (1) of the polarisation sensitive means (32). An apparatus according to any one of the preceding claims, wherein the polarisation sensitive means (32) is a beam splitting means. An apparatus according to any one of the preceding claims, wherein said given polarisation is orthogonal to said another polarisation. An apparatus according to claim 8 wherein said given polarisation is rotated by 90° from said another polarisation. An apparatus according to any one of the preceding claims, wherein the utilisation device (40) is coupled to said third port (4) of the polarisation sensitive means via a polarisation maintaining fibre (36). An apparatus according to any one of the preceding claims, wherein the utilisation device (40) is an interferometer. An apparatus according to claim 11, wherein the interferometer comprises interferometric sensing coil means (60) comprising a polarisation maintaining fibre. An apparatus according to claim 12 when appended to claim 11, comprising an interferometric coupler means (46) having a light path between first and second ports and a light path between third and the first port of the coupler, the interferometric sensing coil having its ends connected to respective ones of the second and third ports of the interferometric coupler means, the interferometric coupler means being coupled to the polarisation maintaining fibre by a polariser (44) bidirectionally transmissive substantially only to pump light of the given polarisation. An apparatus according to any one of claims 11 to 13, wherein the interferometer is a gyro for sensing rotation of the interferometer. An apparatus according to claim 14, and comprising processing circuitry coupled to said second port.

Inventors: FIDRIC BERNARD G, FIDRIC, BERNARD G.
Applicants: LITTON SYSTEMS INC, LITTON SYSTEMS, INC.
Application year: 1993
Main classification: H01S3
IPC codes: H01S3, G02F1, G01C19