Optical source

Abstract

An optical source (100) comprising a primary optical signal source (110), a first optical tap (102) configured to form an optical tap signal, a first fast optical switch (104), a secondary optical signal source (120) and a second fast optical switch (106). Each optical signal source comprises a seed laser (112, 122) configured to generate a seed laser signal and spectral broadening apparatus (114, 124) to broaden the seed laser signal to output a broadened laser signal. The optical source further comprises an optical fibre amplifier (108), an optical combiner (128), optical monitoring apparatus (118) and control circuitry (126). The optical monitoring apparatus is configured to output an alarm signal responsive to a determination that the optical tap signal is indicative of a fault in the spectral broadening apparatus of the primary optical signal source resulting in insufficient broadening of the seed laser signal. The control circuitry (126) is configured to output: an initial control signal to cause the first fast optical switch to pass optical signals and the second fast optical switch to block optical signals; and a switching control signal, responsive to receiving the first alarm signal, to cause the first fast optical switch to block optical signals and to cause the second fast optical switch to pass optical signals.

Description

[0001] OPTICAL SOURCE

[0002] Technical Field

[0003] The disclosure relates to an optical source. The disclosure further relates to a high energy laser processing system comprising the optical source. The disclosure further relates to a laser directed energy weapon comprising the optical source.

[0004] Background

[0005] Narrow linewidth, high power (e.g. kW) laser sources, such as those used for high energy laser processing or laser directed energy, typically comprise a laser source and one or more high power fibre amplifiers to amplify a seed laser signal up to the required operating optical power. Suitable laser sources often consist of a very narrow linewidth seed laser and a spectral broadening module to increase the linewidth in order to mitigate stimulated Brillouin scattering (SBS) in the fibre amplifier. High power fibre amplifiers, such as Ytterbium doped fibre amplifiers, capable of amplifying up to kW optical powers are very vulnerable to damage in the case of a failure of the any of the components of the spectral broadening module. A fault in the spectral broadening module can result in the seed laser being passed unbroadened into the fibre amplifier, and the resulting pulsing due to SBS can cause catastrophic damage to the fibre amplifier in a very short time (the timescale for build up of SBS is approximately 10 ns). The SBS also produces backward travelling pulses which can damage many components of a laser system, all the way back to the seed laser.

[0006] Summary

[0007] It is an object to provide an improved optical source suitable for high power applications. It is a further object to provide an improved high energy laser processing system. It is a further object to provide an improved laser directed energy weapon.

[0008] An aspect provides an optical source comprising a primary channel, a secondary channel, an optical fibre amplifier, an optical combiner, optical monitoring apparatus and control circuitry. The primary channel comprises a primary optical signal source, a first optical tap and a first fast optical switch. The first optical tap is downstream of the primary optical signal source. The first optical tap is configured to form a first optical tap signal. The first fast optical switch is at an output end of the primary channel. The secondary channel comprises a secondary optical signal source and a second fast optical switch at an output end of the secondary channel. Each of the primary optical signal source and the secondary optical signal source comprises a seed laser configured to generate a seed laser signal having a seed optical spectrum having a seed spectral linewidth and spectral broadening apparatus configured to broaden the seed laser signal to output a broadened laser signal having a broadened spectral linewidth, broader than the seed spectral linewidth. The optical fibre amplifier is configured to amplify a said broadened laser signal. The optical combiner is configured to couple the primary channel and the secondary channel to the optical fibre amplifier. The optical monitoring apparatus is configured to receive the first optical tap signal and is configured to output a first alarm signal responsive to a determination that the first optical tap signal is indicative of a fault in the spectral broadening apparatus of the primary optical signal source resulting in insufficient broadening of the seed laser signal. The control circuitry is configured to output at least one initial control signal configured to cause the first fast optical switch to pass optical signals and the second fast optical switch to block optical signals. The control circuitry is further configured to output at least one switching control signal, responsive to receiving the first alarm signal. The at least one switching control signal is configured to cause the first fast optical switch to be configured to block optical signals and to cause the second fast optical switch to be configured to pass optical signals.

[0009] The optical source may enable protection of its optical fibre amplifier from damage due to SBS in the event of a fault in the spectral broadening apparatus of the primary optical signal source resulting in insufficient broadening of the seed laser signal. The optical source may also enable fast switching to its secondary optical signal source in the event of a fault in the spectral broadening apparatus of the primary optical signal source, to enable continued delivery of an amplified, broadened laser signal. The optical source therefore advantageously provides improved protection of one of its most expensive components and improved continuity of operation for a user.

[0010] In an embodiment the spectral broadening apparatus comprises radio frequency, RF, drive signal apparatus and a phase modulator. The RF drive signal apparatus is configured to generate an RF drive signal having an RF signal power. The phase modulator is configured to apply phase modulation to the seed laser signal when an RF drive signal is received to form a broadened laser signal having a broadened optical spectrum having the broadened spectral linewidth. The phase modulator is further configured to output the seed laser signal unbroadened when an RF drive signal is not received. The optical monitoring apparatus is configured to output a first alarm signal responsive to a determination that the first optical tap signal is indicative of a fault in at least one of the RF drive signal apparatus or the phase modulator of the primary optical signal source resulting in insufficient broadening of the seed laser signal. This may enable protection of the optical fibre amplifier from damage due to SBS in the event of a fault in the RF drive signal apparatus and / or the phase modulator.

[0011] In an embodiment, the first fast optical switch is a first semiconductor optical amplifier, SOA, and the second fast optical switch is a second SOA. The at least one initial control signal is configured to cause the first SOA to be configured to amplify optical signals and to cause the second SOA to be configured to absorb optical signals. The at least one switching control signal is configured to cause the first SOA to be configured to absorb optical signals and to cause the second SOA to be configured to amplify optical signals. Using SOAs as the fast optical switches may enable pre-amplification of the broadened laser signal before it is delivered to the optical fibre amplifier, which may enable a higher output optical power to be achieved. Amplification with SOAs can also reduce relative intensity noise, RIN, which is beneficial for high power amplification and coherent combination. Using SOAs as the fast optical switches may enable the seed lasers of both channels to be permanently running during operation of the optical source, enabling fast switching from the primary channel to the secondary channel in the event of a fault in the spectral broadening apparatus and improved continuity of operation.

[0012] In an embodiment, the primary optical channel further comprises an optical delay line between the first optical tap and the first fast optical switch. This advantageously reduces the length of time during which an insufficiently broadened seed signal may be delivered to the optical fibre amplifier before the first fast optical switch is configured to block optical signals, which may mitigate the build up of stimulated Brillouin scattering, SBS, in the optical fibre amplifier.

[0013] In an embodiment, the optical delay line is configured to apply a delay greater than a time required for the optical monitoring apparatus to determine that the first optical tap signal is indicative of a fault in the spectral broadening apparatus and for the first fast optical switch to be configured to block optical signals. This advantageously increases the length of time for completing the reconfiguration of the fast optical switches so that it is longer than the total of the time required to detect a fault, the time to transmit the alarm signal and the control signal, and the switching or rise time of the fast optical switch. This may ensure that the first fast optical switch is configured to block optical signals before an insufficiently broadened seed signal reaches the first fast optical switch, which advantageously prevents an insufficiently broadened seed signal being delivered to the optical fibre amplifier and thus prevents SBS occurring in the optical fibre amplifier.

[0014] In an embodiment, the optical fibre amplifier comprises an active optical fibre and a pump laser.

[0015] In an embodiment, the control circuitry is configured to output a first switching control signal responsive to receiving the first alarm signal. The first switching control signal is configured to cause the first fast optical switch to be configured to block optical signals. The control circuitry is configured to output a second switching control signal configured to cause the pump laser to switch off. The control circuitry is configured to output a third switching control signal configured to cause the second fast optical switch to be configured to pass optical signals. The control circuitry is configured to output the third switching control signal subsequently to the first switching control signal and the second switching control signal.

[0016] This may enable there to be a delay between configuring the first fast optical switch to block optical signals and configuring the second fast optical switch to pass optical signals, without self-lasing occurring in the optical fibre amplifier.

[0017] In an embodiment, the secondary optical channel further comprises a second optical tap downstream of the secondary optical signal source configured to form a second optical tap signal and second optical monitoring apparatus. The second optical monitoring apparatus is configured to receive the second optical tap signal and is configured to output a second alarm signal responsive to a determination that the second optical tap signal is indicative of a fault in the spectral broadening apparatus of the secondary optical signal source resulting in insufficient broadening of the seed laser signal. The control circuitry is further configured to output at least one second switching control signal, responsive to receiving the second alarm signal, configured to cause the second fast optical switch to be configured to block optical signals and to cause the first fast optical switch to be configured to pass optical signals. This may enable protection of its optical fibre amplifier from damage due to SBS in the event of a fault in the spectral broadening apparatus of the secondary optical signal source resulting in insufficient broadening of the seed laser signal.

[0018] In an embodiment, the optical monitoring apparatus comprises optical filter apparatus, a photodetector and monitoring control circuitry. The optical filter apparatus has a transmission profile configured to form an optical monitoring signal by modifying an optical power of a received optical tap signal based on a spectral linewidth of the optical tap signal. The photodetector is configured to detect the optical monitoring signal and to output a detection signal indicative of the optical power of the optical monitoring signal. The monitoring control circuitry is configured to receive the detection signal and to output a said alarm signal responsive to the optical monitoring signal having an optical power indicative of a spectral linewidth indicative of a fault in the spectral broadening apparatus of the primary optical source resulting in insufficient broadening of the seed laser signal. The optical monitoring apparatus advantageously enables the spectral linewidth of an optical tap signal to be represented and detected as an optical power, enabling a simple, low cost and rugged way to monitor the spectral linewidth of the corresponding optical signal generated by the primary channel, with a fast speed of response.

[0019] In an embodiment, the monitoring control circuitry is configured to output a said alarm signal responsive to the optical monitoring signal having an optical power indicative of a spectral linewidth indicative of a fault in at least one of the RF drive signal apparatus or the phase modulator of the primary optical signal source resulting in insufficient broadening of the seed laser signal. This may enable a fault in either component of the spectral broadening apparatus to be identified and a resulting alarm signal to be output.

[0020] In an embodiment, the monitoring control circuitry is configured to compare a power level of the detection signal to a threshold power level indicative of a spectral linewidth indicative of normal operation of the spectral broadening apparatus and to output a said alarm signal if the detection signal has a power level lower than the threshold power level. This may allow for the protection of optical fibre amplifier from damage due to SBS in the event of a fault in the spectral broadening apparatus of the primary optical signal source causing the detection signal power level to fall below the threshold power level.

[0021] In an embodiment, the optical filter apparatus comprises a band-stop optical filter having an optical stop band configured to reject wavelengths corresponding to the seed optical spectrum. This may enable the band-stop optical filter to block any unbroadened seed laser signal (which may remain during normal operation or which is the only signal present as a result of a fault in the spectral broadening apparatus) in the optical monitoring signal while transmitting as much of the optical monitoring signal as possible during normal operation.

[0022] In an alternative embodiment, the optical filter apparatus comprises a band-stop optical filter having an optical stop band configured to reject wavelengths corresponding to 10% to 100% of a full width half maximum, FWHM, of the broadened optical spectrum for normal operation of the spectral broadening apparatus. This may enable improved sensitivity to a reduction in spectral broadening. This may enable a slow failure of the spectral broadening apparatus to be detected, which may enable a reduction in spectral linewidth of a broadened laser signal to be detected before it becomes large enough to cause significant SBS in the optical fibre amplifier.

[0023] In an embodiment, the band-stop optical filter optical stop band is configured to reject wavelengths corresponding to 10% to 30% of the FWHM of the broadened optical spectrum for normal operation of the spectral broadening apparatus. This may enable good contrast between optical monitoring signal power levels for a broadened laser signal and an unbroadened seed laser signal.

[0024] In an embodiment, the band-stop optical filter has a reflection bandwidth corresponding to 30% to 100% of the FWHM of the broadened optical spectrum for normal operation of the spectral broadening apparatus. This may enable further improved sensitivity to a reduction in spectral broadening, This may enable a gradual reduction in the spectral linewidth of a broadened laser signal caused by a slow failure of the spectral broadening apparatus to be detected. Since the optical monitoring signal mainly or entirely comprises only parts of the broadened optical spectrum outside the FHWM (sometimes referred to as the ‘wings’ of an optical spectrum), the optical monitoring apparatus is advantageously highly sensitive to any small reduction in the spectral linewidth of a broadened laser signal.

[0025] In an embodiment, optical monitoring apparatus comprises optical linewidth measurement apparatus configured to determine a linewidth of a received optical tap signal and configured to output a said alarm signal responsive to a determination that the optical tap signal has a spectral linewidth indicative of a fault in the spectral broadening apparatus of the primary optical source resulting in insufficient broadening of the seed laser signal. The optical linewidth measurement apparatus advantageously enables an alarm signal to be generated based on a direct measurement of the linewidth.

[0026] In an embodiment, the optical linewidth measurement apparatus comprises one of an optical spectrum analyser and optical interferometer based linewidth measurement apparatus. This may enable a very accurate measurement of the linewidth, which may mitigate risk of a false detection of a fault in the spectral broadening apparatus.

[0027] In an embodiment, the seed laser signal has a seed optical power in the range 1 mW to 200 mW. The optical source comprises a plurality of optical fibre amplifiers configured to amplify a said broadened laser signal. The optical fibre amplifiers are arranged in series and are configured to amplify said broadened laser signal to an optical power of up to 5 kW.

[0028] In an embodiment, the optical source comprises a plurality of optical amplifiers as described above and an optical splitter. The optical splitter is configured to split a broadened laser signal received from the optical combiner to form a plurality of split broadened laser signals and to route the split broadened laser signals to respective optical amplifiers. This may enable multiple output signals to be formed, which may be combined to form a high power output.

[0029] In an embodiment, each optical fibre amplifier comprises an active optical fibre and a pump laser. The at least one switching control signal is further configured to cause each pump laser to switch off. In an embodiment, the seed lasers are distributed feedback, DFB, fibre lasers.

[0030] In an alternative embodiment, the seed lasers are one of bulk nonplanar ring oscillators, NPRO, DFB diode lasers, distributed Bragg reflector, DBR, diode lasers, or external cavity diode lasers.

[0031] Corresponding embodiments and advantages apply to the high energy laser processing system and to the laser directed energy weapon described below.

[0032] A high energy laser processing system comprising an optical source. The optical source comprises a primary channel, a secondary channel, an optical fibre amplifier, an optical combiner, optical monitoring apparatus and control circuitry. The primary channel comprises a primary optical signal source, a first optical tap and a first fast optical switch. The first optical tap is downstream of the primary optical signal source configured to form a first optical tap signal. The first fast optical switch is at an output end of the primary channel. The secondary channel comprises a secondary optical signal source and a second fast optical switch at an output end of the secondary channel. Each of the primary optical signal source and the secondary optical signal source comprises a seed laser configured to generate a seed laser signal having a seed optical spectrum having a seed spectral linewidth and spectral broadening apparatus configured to broaden the seed laser signal to output a broadened laser signal having a broadened optical spectrum having a broadened spectral linewidth, broader than the seed spectral linewidth. The optical fibre amplifier is configured to amplify a said broadened laser signal. The optical combiner is configured to couple the primary channel and the secondary channel to the optical fibre amplifier. The optical monitoring apparatus is configured to receive the first optical tap signal and is configured to output a first alarm signal responsive to a determination that the first optical tap signal is indicative of a fault in the spectral broadening apparatus of the primary optical signal source resulting in insufficient broadening of the seed laser signal. The control circuitry is configured to output at least one initial control signal configured to cause the first fast optical switch to pass optical signals and the second fast optical switch to block optical signals. The control circuitry is further configured to output at least one switching control signal, responsive to receiving the first alarm signal. The at least one switching control signal is configured to cause the first fast optical switch to be configured to block optical signals and to cause the second fast optical switch to be configured to pass optical signals.

[0033] A laser directed energy weapon comprising an optical source. The optical source comprises a primary channel, a secondary channel, an optical fibre amplifier, an optical combiner, optical monitoring apparatus and control circuitry. The primary channel comprises a primary optical signal source, a first optical tap and a first fast optical switch. The first optical tap is downstream of the primary optical signal source configured to form a first optical tap signal. The first fast optical switch is at an output end of the primary channel. The secondary channel comprises a secondary optical signal source and a second fast optical switch at an output end of the secondary channel. Each of the primary optical signal source and the secondary optical signal source comprises a seed laser configured to generate a seed laser signal having a seed optical spectrum having a seed spectral linewidth and spectral broadening apparatus configured to broaden the seed laser signal to output a broadened laser signal having a broadened optical spectrum having a broadened spectral linewidth, broader than the seed spectral linewidth. The optical fibre amplifier is configured to amplify a said broadened laser signal. The optical combiner is configured to couple the primary channel and the secondary channel to the optical fibre amplifier. The optical monitoring apparatus is configured to receive the first optical tap signal and is configured to output a first alarm signal responsive to a determination that the first optical tap signal is indicative of a fault in the spectral broadening apparatus of the primary optical signal source resulting in insufficient broadening of the seed laser signal. The control circuitry is configured to output at least one initial control signal configured to cause the first fast optical switch to pass optical signals and the second fast optical switch to block optical signals. The control circuitry is further configured to output at least one switching control signal, responsive to receiving the first alarm signal. The at least one switching control signal is configured to cause the first fast optical switch to be configured to block optical signals and to cause the second fast optical switch to be configured to pass optical signals.

[0034] Embodiments of the disclosure will now be described, by way of example only, with reference to the accompanying drawings.

[0035] Brief Description of the drawings

[0036] Figures 1 to 4 and 6 to 12 are block diagrams illustrating embodiments of optical sources; and

[0037] Figure 5 shows plots of: (A) variation of the linewidth of an optical signal output from the primary optical channel at the monitoring apparatus and at the first SOA of Figure 4; (B) drive currents to the first SOA and the second SOA of Figure 4; and the corresponding variation output optical power of the optical source of Figure 4.

[0038] Detailed description

[0039] The same reference numbers are used for corresponding features in different embodiments.

[0040] Referring to Figure 1 , an embodiment provides an optical source 100 comprising a primary channel, a secondary channel, an optical fibre amplifier 108, an optical combiner 128, an optical tap 102, optical monitoring apparatus 118 and control circuitry 126.

[0041] The primary channel comprises a primary optical signal source 110, which comprises a first seed laser 112 and a first spectral broadening apparatus 114. The first seed laser 112 is configured to generate a first seed laser signal having a seed wavelength and a seed optical spectrum having a seed spectral linewidth. The first spectral broadening apparatus 114 is configured to broaden the first seed laser signal to output a first broadened laser signal having a broadened optical spectrum having a broadened spectral linewidth, broader than the seed spectral linewidth, during normal, fault free operation.

[0042] The optical tap 102 is provided downstream of the output of the first spectral broadening apparatus 114, for splitting off a portion of an optical signal output from the first spectral broadening apparatus to form a first optical tap signal. A first fast optical switch 104 is provided at an output end of the primary channel.

[0043] The secondary channel comprises a secondary optical signal source 120, which comprises a second seed laser 122 and a second spectral broadening apparatus 124. The second seed laser 142 is configured to generate a second seed laser signal having substantially the same seed wavelength, seed optical spectrum and seed spectral linewidth as the first seed laser, ‘substantially the same’ here means similar enough to the first seed laser signal that one can replace the other and the optical fibre amplifier will amplify the second broadened laser signal to substantially the same optical power as the first broadened laser signal, i.e. to an optical power within operating tolerances of a downstream apparatus that the optical source is being used with. It will be appreciated by the skilled person that it is typically not practically achievable for the second seed laser to generate a second seed laser signal that is exactly the same as the first seed laser signal. It will also be appreciated that if the optical amplifier 108 is configured to operate in gain saturation, replacing the first seed laser signal with a second seed laser signal that is substantially the same will result an amplified second broadened laser signal having the same optical power as the amplified first broadened laser signal it is replacing.

[0044] The second spectral broadening apparatus 124 is configured to broaden the second seed laser signal to output a second broadened laser signal having substantially the same broadened spectral linewidth as the first broadened laser signal has during normal, fault free operation of the first spectral broadening apparatus 114. A second fast optical switch 106 is provided at an output end of the secondary channel.

[0045] The optical combiner 128 is configured to couple the primary channel to the optical fibre amplifier, so that a first broadened laser signal output from the first spectral broadening apparatus 1 14 is delivered to the optical fibre amplifier. The optical combiner 128 is additionally configured to couple the secondary channel to the optical fibre amplifier, so that a second broadened laser signal output from the second spectral broadening apparatus 1 14 is delivered to the optical fibre amplifier. The optical fibre amplifier 108 is configured to amplify a first broadened laser signal output from the primary optical channel or a second broadened laser signal output from the secondary optical channel.

[0046] The optical monitoring apparatus 118 is configured to receive the first optical tap signal. The optical monitoring apparatus is configured to output a first alarm signal responsive to a determination that the first optical tap signal is indicative of a fault in the first spectral broadening apparatus 114 resulting in insufficient broadening of the first seed laser signal. Spectral broadening is considered to be insufficient if the broadened optical spectrum has a broadened spectral linewidth at which SBS is likely to occur in the optical fibre amplifier. The control circuitry 126 is configured to output at least one initial control signal configured to cause the first fast optical switch 104 to pass optical signals and the second fast optical switch 106 to block optical signals. The control circuitry 126 is also configured to output a switching control signal, responsive to receiving a first alarm signal from the optical monitoring apparatus. The switching control signal is configured to cause the first fast optical switch to be configured to block optical signals and to cause the second fast optical switch to be configured to pass optical signals. Alternatively, two switching control signals may be output by the control circuitry, one for the first fast optical switch 104 and a second for the second fast optical switch 106.

[0047] During normal, fault free operation, both the primary channel and the secondary channel are operational. The first seed laser 112 generates a first seed laser signal and the first spectral broadening apparatus 114 broadens the first seed laser signal to output a first broadened laser signal having a broadened optical spectrum having a broadened spectral linewidth, broader than the seed spectral linewidth. The second seed laser 122 generates a second seed laser signal and the second spectral broadening apparatus 124 broadens the second seed laser signal to output a second broadened laser signal having a broadened optical spectrum having the broadened spectral linewidth. The control circuitry 126 outputs an initial control signal configured to cause the first fast optical switch 104 to pass optical signals and the second fast optical switch 106 to block optical signals. The first broadened laser signal is therefore delivered, via the optical combiner 1 16, to the optical fibre amplifier 108.

[0048] If a fault occurs in the first spectral broadening apparatus 114 which results in insufficient broadening of the first seed laser signal, this will be detected by the optical monitoring apparatus and a first alarm signal will be output. A fault in the first spectral broadening apparatus may result in the first seed laser signal still being broadened but the amount of broadening not being sufficient to mitigate SBS in the optical fibre amplifier. A fault in the first spectral broadening apparatus may alternatively result in the first seed laser signal not being broadened at all, i.e. the first seed laser signal is simply passed through the first spectral broadening apparatus. An unbroadened seed laser signal should preferably not be delivered to the optical fibre amplifier, since it may result in SBS within the optical fibre amplifier. In both cases, the control circuitry 126 will output a switching control signal responsive to receiving a first alarm signal from the optical monitoring apparatus. The switching control signal is configured to cause the first fast optical switch 104 to be configured to block optical signals and to cause the second fast optical switch 106 to be configured to pass optical signals.

[0049] Following reconfiguration of the fast optical switches, the second broadened laser signal is delivered to the optical fibre amplifier 108, enabling the optical source 100 to continue operating. The first spectral broadening apparatus 114 may then be repaired or replaced, without affecting continuing operation of the optical source.

[0050] In an embodiment, the first seed laser 112 and the second seed laser 122 are DFB fibre lasers.

[0051] In an alternative embodiment, the first seed laser 112 and the second seed laser 122 are bulk nonplanar ring oscillators (NPRO), DFB diode lasers, DBR diode lasers, or external cavity diode lasers.

[0052] Referring to Figure 1A, an embodiment provides an optical source 150 comprising a primary channel, a secondary channel, a first optical fibre amplifier 108, an optical combiner 128, an optical tap 102, optical monitoring apparatus 118 and control circuitry 126, as described above. The optical source additionally comprises a second optical fibre amplifier 158.

[0053] The first seed laser signal has a seed optical power in the range 1 mW to 200 mW. The first optical fibre amplifier 108 and second optical fibre amplifier 158 are arranged in series and are configured to amplify the first broadened laser signal or the second broadened laser signal to an optical power of up to 5 kW. It will be appreciated that more than two optical fibre amplifiers in series may be used. At least the second optical fibre amplifier 158 may be configured to operate in gain saturation. Referring to Figure 2, an embodiment provides an optical source 200 comprising a primary channel, a secondary channel, an optical fibre amplifier 208, an optical combiner 128, an optical tap 102, optical monitoring apparatus 218 and control circuitry 126.

[0054] The primary channel comprises a primary optical signal source 210, which comprises a first seed laser 112 and a first spectral broadening apparatus. The first seed laser 112 is configured to generate a first seed laser signal having a seed wavelength and having a seed optical spectrum having a seed spectral linewidth. The first spectral broadening apparatus comprises a first radio frequency, RF, drive signal apparatus 212 and a first phase modulator, PM, 214. The first RF drive signal apparatus is configured to generate a first RF drive signal having an RF signal power, for the first PM. The first PM 214 is configured to apply phase modulation to the first seed laser signal, when a first RF drive signal is received from the first RF drive signal apparatus, to form a first broadened laser signal having a broadened optical spectrum having a broadened spectral linewidth, broader than the seed spectral linewidth. The first PM 214 is configured to output the first seed laser signal unbroadened when no first RF drive signal is received.

[0055] The secondary channel comprises a secondary optical signal source 220, which comprises a second seed laser 122 and a second spectral broadening apparatus. The second seed laser 122 is configured to generate a second seed laser signal having a seed wavelength and a seed optical spectrum having a seed spectral linewidth. The second spectral broadening apparatus comprises a second radio frequency, RF, drive signal apparatus 222 and a second phase modulator, PM, 224. The second RF drive signal apparatus is configured to generate a second RF drive signal having an RF signal power, for the second PM. The second PM 224 is configured to apply phase modulation to the second seed laser signal, when a second RF drive signal is received from the second RF drive signal apparatus, to form a second broadened laser signal having a broadened optical spectrum having the broadened spectral linewidth. The second PM 224 is configured to output the second seed laser signal unbroadened when no second RF drive signal is received.

[0056] The optical fibre amplifier 208 comprises an active optical fibre 202 and a pump laser 204.

[0057] The optical monitoring apparatus 218 is configured to receive the first optical tap signal. The optical monitoring apparatus 218 is configured to output a first alarm signal responsive to a determination that the first optical tap signal is indicative of a fault in the first RF drive signal apparatus 212 and / or the first PM 214 resulting in insufficient broadening of the first seed laser signal.

[0058] The control circuitry 126 is configured to output at least one initial control signal configured to cause the first fast optical switch 104 to pass optical signals and the second fast optical switch 106 to block optical signals. The control circuitry is also configured to output a switching control signal, responsive to receiving a first alarm signal from the optical monitoring apparatus. The switching control signal is configured to cause the first fast optical switch 104 to be configured to block optical signals and to cause the second fast optical switch 106 to be configured to pass optical signals. Alternatively, two switching control signals may be output by the control circuitry, one for the first fast optical switch and a second for the second fast optical switch.

[0059] The optical combiner 128 is configured to couple the primary channel to the optical fibre amplifier and to couple the secondary channel to the optical fibre amplifier, as described above with reference to Figure 1 .

[0060] Referring to Figure 3, an embodiment provides an optical source 300 comprising a primary channel, a secondary channel, an optical fibre amplifier 208, an optical combiner 128, an optical tap 102 and optical monitoring apparatus 218, as described above with reference to Figures 1 and 2, and control circuitry 326.

[0061] A first fast optical switch 304 is provided at an output end of the primary channel and a second fast optical switch 306 is provided at an output end of the secondary channel. The first fast optical switch is a first semiconductor optical amplifier, SOA, 304. The second fast optical switch is a second SOA 306.

[0062] The control circuitry 326 is configured to output at least one initial control signal configured to cause the first SOA 304 to be configured to amplify optical signals and to cause the second SOA 306 to be configured to absorb optical signals.

[0063] The control circuitry is also configured to output a first switching control signal 310, responsive to receiving a first alarm signal from the optical monitoring apparatus. The first switching control signal is configured to cause the first SOA to be configured to absorb optical signals. The control circuitry is further configured to output a second switching control signal 312 configured to cause the optical amplifier pump laser 204 to switch off. The control circuitry is further configured to output a third switching control signal 314 configured to cause the second SOA to be configured to amplify optical signals. The second switching control signal may be output at the same time as the first switching control signal, or within a time period shorter than the time period for self-lasing to occur within the optical fibre 202, which is typically of the order of milliseconds. The third switching control signal is output subsequently to the first switching control signal and the second switching control signal, and the delay before outputting the third switching control signal may be pre-set or manually controlled.

[0064] The SOAs may, for example, be an Innolume SOA-1060-20-YY-40dB which absorbs approximately 50 dB and can amplify a 1 mW seed laser signal to 150 mW optical power.

[0065] In an embodiment, illustrated in Figure 4, the optical source 400 additionally comprises an optical delay line 402 in the primary optical channel. The optical delay line is provided between the first optical tap 102 and the first SOA 304.

[0066] In an embodiment, the optical delay line 402 is configured to apply a delay greater than a time required for the optical monitoring apparatus 218 to determine that the first optical tap signal is indicative of a fault in the first PM 214 and / or the first RF drive signal apparatus 212, and for the first SOA 304 to be configured to absorb optical signals.

[0067] For example, the optical delay line may be a 20 m long optical fibre, to introduce a 100 ns delay. Figure 5 illustrates the switching of the first SOA 304 and the second SOA 306 in response to a detected reduction in the spectral linewidth of the optical signal output from the first PM 214, and the resulting effect on the output optical power of the optical source 400.

[0068] Figure 5(A) shows the linewidth 410 detected at the optical monitoring apparatus and the linewidth 412 at the first SOA. It shows the linewidth falling below a preset threshold at time t1 . It takes at time At for the optical monitoring apparatus 218 to determine that the first optical tap signal is indicative of a fault in the first PM 214, for example, and for it to transmit the alarm signal, and for the control circuitry to output a first switching control signal to the first SOA 304 and a second switching control signal to the second SOA 306. As shown in Figure 5(B), in response to receiving the first switching control signal, the first SOA drive current 414 is reduced and in response to receiving the second switching control signal, the second SOA drive current 416 is increased. As shown in Figure 5(C), during the switch from the first SOA to the second SOA, the optical power output falls and then increases back to the previous level.

[0069] In an embodiment, illustrated in Figure 6, the optical source 600 secondary optical channel further comprises a second optical tap 602 configured to form a second optical tap signal and second optical monitoring apparatus 618. The second optical tap is provided downstream of the second PM 224. The second optical monitoring apparatus is configured to receive the second optical tap signal and is configured to output a second alarm signal responsive to a determination that the second optical tap signal is indicative of a fault in the second RF drive signal apparatus 222 and / or the second PM 224 resulting in insufficient broadening of the seed laser signal.

[0070] In this embodiment, the control circuitry 626 is configured as described above and is additionally configured to output a second switching control signal, responsive to receiving the second alarm signal, configured to cause the second SOA to be configured to absorb optical signals and to cause the first SOA to be configured to amplify optical signals.

[0071] Referring to Figure 7, an embodiment provides an optical source 700 comprising a primary channel, a secondary channel, an optical fibre amplifier 208, an optical combiner 128, an optical tap 102 and control circuitry 126 as described above with reference to Figure 1 , and optical monitoring apparatus 718.

[0072] The optical monitoring apparatus 718 comprises optical filter apparatus 702, a photodetector 704 and monitoring control circuitry 706. The optical filter apparatus is configured to receive the optical tap signal from the optical tap. The optical filter apparatus has a transmission profile configured to modify an optical power of the optical tap signal based on its spectral linewidth, to form an optical monitoring signal having an optical power indicative of the spectral linewidth of the optical tap signal. The photodetector is configured to detect the optical monitoring signal and to output a detection signal indicative of the optical power of the optical monitoring signal. The monitoring control circuitry is configured to receive the detection signal and to output an alarm signal responsive to the detection signal being indicative of a fault in the first spectral broadening apparatus 114 resulting in insufficient broadening of the seed laser signal. The detection signal is indicative of the optical power of the monitoring signal, which is indicative of the spectral linewidth of the optical tap signal and thus is indicative of the linewidth of the optical signal output from the first spectral broadening apparatus 114. If the linewidth is insufficiently broad, that is indicative of a fault in the first spectral broadening apparatus 114, as discussed above with reference to Figure 1 .

[0073] In an embodiment, the monitoring control circuitry 706 is configured to compare a power level of the detection signal to a threshold power level. The threshold power level is indicative of a broadened optical spectrum having a broadened spectral linewidth indicative of normal operation of the spectral broadening apparatus. The monitoring control circuitry 706 is configured to output an alarm signal if the detection signal has a power level lower than the threshold power level.

[0074] Referring to Figure 8, an embodiment provides an optical source 800 comprising a primary channel, a secondary channel, an optical fibre amplifier 208, an optical combiner 128, an optical tap 102 and control circuitry 126 as described above with reference to Figure 2, and optical monitoring apparatus 818.

[0075] The primary channel comprises a primary optical signal source 210, which comprises a first seed laser 112 and a first spectral broadening apparatus comprising a first PM 214 and a first RF drive signal source 212, as described above with reference to Figure 2. The first seed laser 112 is configured to generate a first seed laser signal having a seed wavelength and a seed optical spectrum having a seed spectral linewidth, as shown in inset (a) of Figure 9. The first PM 214 is configured to apply phase modulation to the first seed laser signal, when a first RF drive signal is received from the first RF drive signal apparatus, to form a first broadened laser signal having a broadened optical spectrum having a broadened spectral linewidth, broader than the seed spectral linewidth, as shown in inset (b) of Figure 9.

[0076] The secondary channel comprises a secondary optical signal source 220, which comprises a second seed laser 122 and a second spectral broadening apparatus comprising a second PM 224 and a second RF drive signal source 222, as described above with reference to Figure 2.

[0077] The optical monitoring apparatus 718 comprises a band-stop optical filter, BSF, 802, a photodetector 704 and monitoring control circuitry 706. The band-stop optical filter is configured to receive the optical tap signal from the optical tap. The band-stop optical filter has an optical stop band configured to reject wavelengths corresponding to the seed optical spectrum; the transmission profile of the band-stop optical filter is shown in inset (c) of Figure 8.

[0078] The band-stop optical filter has very high reflectivity at wavelengths corresponding to the seed optical spectrum and transmits other wavelengths of the optical tap signal The band-stop optical filter modifies an optical power of the optical tap signal based on its spectral linewidth, to form an optical monitoring signal having an optical power indicative of the spectral linewidth of the optical tap signal. This is because transmission through the band-stop optical filter forms a notch in the optical spectrum of an optical tap signal from a first broadened laser signal, as illustrated in inset (d) of Figure 8. The band- stop optical filter may, for example, be a fibre Bragg grating, FBG having a reflectivity in the range 90 - 99% at wavelengths corresponding to the seed optical spectrum.

[0079] In operation, if the RF signal power of the RF drive signal falls, as a result of a fault with the first RF drive signal apparatus 212, the amount of spectral broadening performed by the first PM 214 will be correspondingly reduced. The total optical power of the optical monitoring signal will therefore fall since the band-stop optical filter will still filter out the same wavelengths but these will be a larger proportion of the optical spectrum of the optical tap signal. If there is a fault with the first RF drive signal apparatus 212 that results in no RF drive signal being generated, or if there is a fault with the first PM 214 that results in no phase modulation occurring, the first seed laser signal will pass through the first PM 214 unbroadened. Since the band-stop optical filter has very high reflectivity at wavelengths corresponding to the seed optical spectrum, very little of an unbroadened first seed laser signal will be transmitted by the band-stop optical filter, meaning the optical monitoring signal will have very low total optical power. For example, if the band-stop optical filter is an FBG having a reflectivity in the range 90 - 99% only 1- 10% of the first seed laser signal will be transmitted.

[0080] The photodetector is configured to detect the optical monitoring signal and to output a detection signal indicative of the optical power of the optical monitoring signal. The monitoring control circuitry is configured to receive the detection signal and to output an alarm signal responsive to the detection signal being indicative of a fault in the first PM 214 and / or the first RF drive signal apparatus 212 resulting in insufficient broadening of the seed laser signal. The detection signal is indicative of the optical power of the monitoring signal, which is indicative of the spectral linewidth of the optical tap signal and thus is indicative of the spectral linewidth of the optical signal output from the first spectral broadening apparatus 1 14. If the linewidth is insufficiently broad, that is indicative of a fault in the first PM 214 and / or the first RF drive signal apparatus 212, as discussed above.

[0081] Referring to Figure 9, an embodiment provides an optical source 900 comprising a primary channel, a secondary channel, an optical fibre amplifier 208, an optical combiner 128, an optical tap 102 and control circuitry 126 as described above with reference to Figure 2, and optical monitoring apparatus 904.

[0082] The optical monitoring apparatus 904 comprises a photodetector 704 and monitoring control circuitry 706, as described above, and a band-stop optical filter, BSF, 902. The band-stop optical filter is configured to receive the optical tap signal from the optical tap.

[0083] The band-stop optical filter 902 has an optical stop band configured to reject wavelengths corresponding to 10% to 100% of a full width half maximum, FWHM, of the broadened optical spectrum for normal operation of the spectral broadening apparatus. The transmission profile of the band-stop optical filter is shown in inset (c) of Figure 9. The sensitivity of the optical monitoring apparatus 904 may be increased by increasing the width of the band-stop optical filter stop band relative to the broadened spectral linewidth of the broadened optical spectrum. The band-stop optical filter may, for example, be an FBG used in transmission, having a reflection bandwidth corresponding to 10% to 100% of the FWHM of the broadened optical spectrum.

[0084] In an embodiment, the band-stop optical filter optical stop band is configured to reject wavelengths corresponding to 10% to 30% of the FWHM of the broadened optical spectrum of the first broadened laser signal for normal operation of the first spectral broadening apparatus.

[0085] In an embodiment, the band-stop optical filter has a reflection bandwidth corresponding to 30% to 100% of the FWHM of the broadened optical spectrum of the first broadened laser signal for normal operation of the first spectral broadening apparatus.

[0086] Referring to Figure 10, an embodiment provides an optical source 1000 comprising a primary channel, a secondary channel, an optical fibre amplifier 208, an optical combiner 128 and an optical tap 102, as described above with reference to Figures 1 and 2, a first fast optical switch 304, a second fast optical switch 306 and control circuitry 326 as described above with reference to Figure 3, and an optical delay line 402.

[0087] The first fast optical switch 304 may comprise a semiconductor optical amplifier, SOA, and an SOA driver.

[0088] The optical delay line 402 is configured to apply a delay greater than a time required for the optical monitoring apparatus 904 to determine that the first optical tap signal is indicative of a fault in the first PM 214 and / or the first RF drive signal apparatus 212, and for the first SOA 304 to be configured to absorb optical signals.

[0089] The optical monitoring apparatus 904 comprises a photodetector 704 and monitoring control circuitry 706, and a band-stop optical filter, BSF, 902, as described above with reference to Figure 8.

[0090] For example, the photodetector 704 may have a rise time of 10 ns, the monitoring control circuitry 706 may have processing time of around 10 ns, the control circuitry 326 may have a processing time of around 50 ns, the first SOA 304 driver may have a rise time of 10 ns and a delay of around 10 ns, and the SOA may have a lifetime of 1 ns. The time required for the optical monitoring apparatus 904 to determine that the first optical tap signal is indicative of a fault in the first PM 214 and / or the first RF drive signal apparatus 212, and for the first SOA 304 to be configured to absorb optical signals may therefore be around 90 ns.

[0091] The optical delay line therefore needs to introduce a delay of at least 90 ns. For example, the delay line may be a 20 m long optical fibre, to introduce a 100 ns delay. A delay line comprising 20 m - 40 m of optical fibre will introduce a delay in the range 100 ns to 200 ns. Longer delays may also be used but will require longer lengths of optical fibre, which can have a significant size.

[0092] Referring to Figure 11 , an embodiment provides an optical source 1100 comprising a primary channel, a secondary channel, an optical fibre amplifier 108, an optical combiner 128, an optical tap 102 and control circuitry 126 as described above with reference to Figure 1 , and optical monitoring apparatus 1102. The optical monitoring apparatus comprises optical linewidth measurement apparatus 1102 configured to determine a linewidth of a received optical tap signal. The optical linewidth measurement apparatus is additionally configured to output an alarm signal responsive to a determination that the optical tap signal has a spectral linewidth indicative of a fault in the first spectral broadening apparatus 114 resulting in insufficient broadening of the seed laser signal.

[0093] In an embodiment, the optical linewidth measurement apparatus 1102 comprises a diffraction type optical spectrum analyser.

[0094] In an alternative embodiment, the optical linewidth measurement apparatus 911102 8 comprises optical interferometer based linewidth measurement apparatus.

[0095] The optical interferometer based linewidth measurement apparatus may comprise a reference laser configured to generate a reference laser signal, a high speed photodiode and an electrical spectrum analyser. The optical interferometer based linewidth measurement apparatus is configured to perform heterodyne linewidth measurement. The optical tap signal is mixed with the reference laser signal and the combined laser signal is received and detected by the high speed photodiode. The resulting beat signal produced on a high speed photodiode is measured using the electrical spectrum analyser. The measured electrical spectrum is the convolution of the spectrum of the reference laser signal with that of the optical tap signal.

[0096] Alternatively, the optical interferometer based linewidth measurement apparatus may comprise an optical splitter, an optical delay line, an optical frequency shifter, a high speed photodiode and an electrical spectrum analyser. The optical interferometer based linewidth measurement apparatus is configured to perform delayed self-heterodyne linewidth measurement. The optical tap signal is split in two by the optical splitter, and one half is delayed and frequency shifted. The optical tap signal is mixed with the delayed and frequency shifted replica of itself. The resulting interference signal is centred at the modulation frequency used to frequency shift the delayed replica. The detected electrical spectrum is the convolution of the optical spectrum of the laser signal being measured with itself.

[0097] Referring to Figure 12 an embodiment provides an optical source 1200 comprising a primary channel, a secondary channel, an optical combiner 128, an optical tap 102, optical monitoring apparatus 118, and control circuitry 126, as described above with reference to Figure 1 .

[0098] The optical source 1200 comprises a plurality of optical amplifiers 108, as described above, and an optical splitter 1202. The optical splitter is configured to split a first broadened laser signal or second broadened laser signal received from the optical combiner 128 to form a plurality of split broadened laser signals and to route the split broadened laser signals to respective optical amplifiers 108(1) to 109(n).

[0099] In an embodiment, each optical fibre amplifier 108 comprises an active optical fibre and a pump laser, as described above with reference to Figure 2. The at least one switching control signal is further configured to cause each pump laser to switch off. An embodiment provides high energy laser processing system comprising an optical source 100, 200, 300, 400, 500, 600, 700, 800, 900, 1000, 1100, 1200 as described above with reference to any one of Figures 1 to 4 and 6 to 12.

[0100] An embodiment provides a laser directed energy weapon comprising an optical source 100, 200, 300, 400, 500, 600, 700, 800, 900, 1000, 1100, 1200 as described above with reference to any one of Figures

[0101] 1 to 4 and 6 to 12.

Claims

CLAIMS1. An optical source (100, 150, 200, 300, 400, 500, 600, 700, 800, 900, 1000, 1100, 1200) comprising: a primary channel comprising a primary optical signal source (110), a first optical tap (102) downstream of the primary optical signal source configured to form a first optical tap signal and a first fast optical switch (104, 304) at an output end of the primary channel; a secondary channel comprising a secondary optical signal source (120) and a second fast optical switch (106, 306) at an output end of the secondary channel, wherein each optical signal source comprises: a seed laser (112, 122) configured to generate a seed laser signal having a seed optical spectrum having a seed spectral linewidth; and spectral broadening apparatus (114, 124) configured to broaden the seed laser signal to output a broadened laser signal having a broadened optical spectrum having a broadened spectral linewidth, broader than the seed spectral linewidth; an optical fibre amplifier (108, 208) configured to amplify a said broadened laser signal; an optical combiner (128) configured to couple the primary channel and the secondary channel to the optical fibre amplifier; optical monitoring apparatus (118, 218, 718, 818, 904, 1102) configured to receive the first optical tap signal and configured to output a first alarm signal responsive to a determination that the first optical tap signal is indicative of a fault in the spectral broadening apparatus of the primary optical signal source resulting in insufficient broadening of the seed laser signal; and control circuitry (126, 326, 526, 626) configured to: output at least one initial control signal configured to cause the first fast optical switch to pass optical signals and the second fast optical switch to block optical signals; and output at least one switching control signal, responsive to receiving the first alarm signal, configured to cause the first fast optical switch to be configured to block optical signals and to cause the second fast optical switch to be configured to pass optical signals.

2. The optical source of claim 1 , wherein the spectral broadening apparatus comprises: radio frequency, RF, drive signal apparatus (212, 222) configured to generate an RF drive signal having an RF signal power; and a phase modulator (214, 224) configured to apply phase modulation to the seed laser signal when an RF drive signal is received to form a broadened laser signal having a broadened optical spectrum having the broadened spectral linewidth and configured to output the seed laser signal unbroadened when an RF drive signal is not received, andwherein the optical monitoring apparatus (218, 818) is configured to output a first alarm signal responsive to a determination that the first optical tap signal is indicative of a fault in at least one of the RF drive signal apparatus (212) or the phase modulator (214) of the primary optical signal source (210) resulting in insufficient broadening of the seed laser signal.

3. The optical source of any one of claim 1 or claim 2, wherein the first fast optical switch is a first semiconductor optical amplifier, SOA, (304) and the second fast optical switch is a second SOA (306), and wherein the at least one initial control signal is configured to cause the first SOA to be configured to amplify optical signals and to cause the second SOA to be configured to absorb optical signals, and the at least one switching control signal is configured to cause the first SOA to be configured to absorb optical signals and to cause the second SOA to be configured to amplify optical signals.

4. The optical source of any one of claims 1 to 3, wherein the primary optical channel further comprises an optical delay line (402) between the first optical tap (102) and the first fast optical switch (104, 304).

5. The optical source of claim 4, wherein the optical delay line is configured to apply a delay greater than a time required for the optical monitoring apparatus to determine that the first optical tap signal is indicative of a fault in the spectral broadening apparatus and for the first fast optical switch to be configured to block optical signals.

6. The optical source of any one of claims 1 to 5, wherein the optical fibre amplifier (208) comprises an active optical fibre (202) and a pump laser (204).

7. The optical source of claim 6, wherein the control circuitry is configured to output: a first switching control signal, responsive to receiving the first alarm signal, configured to cause the first fast optical switch to be configured to block optical signals; a second switching control signal configured to cause the pump laser to switch off; and a third switching control signal configured to cause the second fast optical switch to be configured to pass optical signals, wherein the control circuitry is configured to output the third switching control signal subsequently to the first switching control signal and the second switching control signal.

8. The optical source of any one of claims 1 to 7, wherein: the secondary optical channel further comprises a second optical tap (602) downstream of the secondary optical signal source configured to form a second optical tap signal; and second optical monitoring apparatus (618) configured to receive the second optical tap signal and configured to output a second alarm signal responsive to a determination that the second optical tap signal is indicative of a fault in the spectral broadeningapparatus of the secondary optical signal source resulting in insufficient broadening of the seed laser signal, and wherein the control circuitry (626) is further configured to output at least one second switching control signal, responsive to receiving the second alarm signal, configured to cause the second fast optical switch to be configured to block optical signals and to cause the first fast optical switch to be configured to pass optical signals.

9. The optical source of any one of the preceding claims, wherein optical monitoring apparatus (718, 818, 904) comprises: optical filter apparatus (702, 802, 852) having a transmission profile configured to form an optical monitoring signal by modifying an optical power of a received optical tap signal based on a spectral linewidth of the optical tap signal; a photodetector (704) configured to detect the optical monitoring signal and to output a detection signal indicative of the optical power of the optical monitoring signal; and monitoring control circuitry (706) configured to receive the detection signal and to output a said alarm signal responsive to the optical monitoring signal having an optical power indicative of a spectral linewidth indicative of a fault in the spectral broadening apparatus of the primary optical source resulting in insufficient broadening of the seed laser signal.

10. The optical source of claim 9, when dependent on claim 2, wherein the monitoring control circuitry (706) is configured to output a said alarm signal responsive to the optical monitoring signal having an optical power indicative of a spectral linewidth indicative of a fault in at least one of the RF drive signal apparatus (212) or the phase modulator (214) of the primary optical signal source resulting in insufficient broadening of the seed laser signal.

11. The optical source of any one of claim 9 or claim 10, wherein the monitoring control circuitry (706) is configured to: compare a power level of the detection signal to a threshold power level indicative of a broadened optical spectrum having a broadened spectral linewidth indicative of normal operation of the spectral broadening apparatus; and output a said alarm signal if the detection signal has a power level lower than the threshold power level.

12. The optical source of any one of claims 9 to 11 , wherein the optical filter apparatus (702) comprises one of: a band-stop optical filter (802) having an optical stop band configured to reject wavelengths corresponding to the seed optical spectrum; or a band-stop optical filter (852) having an optical stop band configured to reject wavelengths corresponding to 10% to 100% of a full width half maximum, FWHM, of the broadened optical spectrum for normal operation of the spectral broadening apparatus.

13. The optical source of any one of claims 1 to 8, wherein optical monitoring apparatus comprises optical linewidth measurement apparatus (1102) configured to determine a linewidth of a received optical tap signal and configured to output a said alarm signal responsive to a determination that the optical tap signal has a spectral linewidth indicative of a fault in the spectral broadening apparatus of the primary optical source resulting in insufficient broadening of the seed laser signal.

14. The optical source of claim 13, wherein the optical linewidth measurement apparatus (1102) comprises one of an optical spectrum analyser and optical interferometer based linewidth measurement apparatus.

15. The optical source of any one of the preceding claims, wherein the seed laser signal has a seed optical power in the range 1 mW to 200 mW, and wherein the optical source comprises a plurality of optical fibre amplifiers (108, 158) configured to amplify a said broadened laser signal and the optical fibre amplifiers are arranged in series and are configured to amplify the broadened laser signal to an optical power of up to 5 kW.

16. The optical source of any one of the preceding claims, further comprising a plurality of optical amplifiers (108) and an optical splitter (1202) configured to split a said broadened laser signal received from the optical combiner (128) to form a plurality of split broadened laser signals and to route the split broadened laser signals to respective optical amplifiers.

17. The optical source of claim 16, wherein each optical fibre amplifier (108) comprises an active optical fibre and a pump laser, and wherein the at least one switching control signal is further configured to cause each pump laser to switch off.

18. The optical source of any one of the preceding claims, wherein the seed lasers are one of distributed feedback, DFB, fibre lasers, bulk nonplanar ring oscillators, NPRO, DFB diode lasers, distributed Bragg reflector, DBR, diode lasers, or external cavity diode lasers.

19. A high energy laser processing system comprising an optical source (100, 200, 300, 400, 500, 600, 700, 800, 900, 1000, 1100, 1200) according to any one of claims 1 to 18.

20. A laser directed energy weapon comprising an optical source (100, 200, 300, 400, 500, 600, 700, 800, 900, 1000, 1100, 1200) according to any one of claims 1 to 18.

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