Optical source

Abstract

An optical source (100) comprising: a seed laser (102); spectral broadening apparatus (104) configured to broaden the seed laser signal to output a broadened laser signal; an optical tap (106) downstream of the spectral broadening apparatus for forming an optical tap signal; and optical monitoring apparatus (120) comprising: optical filter apparatus (108) configured to receive the optical tap signal and having a transmission profile configured to form an optical monitoring signal by modifying an optical power of the optical tap signal based on a spectral linewidth of the optical tap signal; a photodetector (110) configured to detect the optical monitoring signal and to output a detection signal indicative of the optical power of the monitoring optical signal; and control circuitry (112) 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.

Description

[0001] OPTICAL SOURCE

[0002] Technical Field

[0003] The disclosure relates to an optical source.

[0004] Background

[0005] Narrow linewidth, high power optical sources, such as those used for laser directed energy, typically comprise a seed laser and an optical fibre amplifier. The seed laser generates a relatively low power signal which is then amplified by the optical fibre amplifier to an output signal having an optical power of the order of kilowatts. Often, a seed laser with a particularly narrow linewidth is used. To mitigate the effects of stimulated Brillouin scattering (SBS) in the optical fibre amplifier, the linewidth of the seed laser signal is broadened. One common method to achieve linewidth broadening is the use of phase modulators driven by amplified radio frequency (RF) noise.

[0006] Optical fibre amplifiers capable of amplifying to kilowatt levels are particularly vulnerable to damage in the event of a failure of the seed laser or the linewidth broadening module. Pulsing due to SBS or a lack of seed power can cause catastrophic damage to the amplifier in a short amount of time. Therefore, it is beneficial to monitor the operation of the seed laser and the linewidth broadening module to detect any faults.

[0007] Summary

[0008] It is an object to provide an improved optical source. It is an object to provide an optical source having improved detection of a fault in a linewidth broadening module of the optical source.

[0009] An aspect provides an optical source comprising a seed laser, a spectral broadening apparatus, an optical tap and an optical monitoring apparatus. The seed laser is configured for generating a seed laser signal having a seed laser power and a seed optical spectrum having a seed spectral linewidth. The spectral broadening apparatus is configured to broaden the seed laser signal to output a broadened laser signal having a broadened optical spectrum having a second spectral linewidth, broader than the seed spectral linewidth. The optical tap is provided downstream of the spectral broadening apparatus. The optical tap is configured for forming an optical tap signal. The optical monitoring apparatus comprises an optical filter apparatus, a photodetector and control circuitry. The optical filter apparatus is configured to receive the optical tap signal. The optical filter apparatus has a transmission profile configured to form an optical monitoring signal by modifying an optical power of the 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 monitoring optical signal. The control circuitry is configured to receive the detection signal and to output an 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.

[0010] The optical source may provide a simple, low cost, and rugged approach to monitor the performance of the spectral broadening module of an optical source, in a small and lightweight package. This may allow for the protection of downstream optical devices receiving light from the optical source from damage by preventing them from being enabled, or by preventing insufficiently or unbroadened light being delivered to them, in the event of a fault with the broadening module. For example, the optical source may provide protection of a downstream high power optical fibre amplifier from damage in the event of a fault with the broadening module.

[0011] The optical source may enable a more efficient and cost-effective solution compared to alternative methods of measuring or monitoring laser linewidth using interferometers, which can be relatively complicated, expensive, and susceptible to misalignment or sensitivity to environmental conditions.

[0012] In an embodiment, the spectral broadening apparatus comprises a 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 the second spectral linewidth. The phase modulator is additionally configured to output the seed laser signal unbroadened when an RF drive signal is not received. The control circuitry is configured to output an 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. This may enable automated detection of a fault in the RF drive signal apparatus or the phase modulator. This may allow for the protection of downstream optical devices receiving light from the optical source in the event of a fault in the RF drive signal apparatus or the phase modulator.

[0013] In an embodiment, the control circuitry is configured to compare a power level of the detection signal to a threshold power level. The control circuitry is configured 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 downstream optical devices receiving light from the optical source in the event of a fault in the spectral broadening module causing the detection signal power level to fall below the threshold power level.

[0014] In an embodiment, the threshold power level is set at a power level above a noise level of the photodetector and below a detection signal power level indicative of a second spectral linewidth of the broadened optical spectrum indicative of normal operation of the spectral broadening apparatus. This may be particularly advantageous for detection of a fault resulting in a fast failure of the spectral broadening apparatus. The threshold power level may be set at a power level that additionally takes into account any expected optical monitoring signal variation due to environmental conditions. This may mitigate the occurrence of false alarm signals.

[0015] In an embodiment, the photodetector is a low-noise photodetector and the threshold power level is in a range 80% to 100% of a detection signal power level indicative of a spectral linewidth indicative of normal operation of the spectral broadening apparatus. This may be particularly advantageous for detection of a gradual reduction in the spectral linewidth of the broadened optical spectrum as a result of a fault causing a slow failure of the spectral broadening apparatus. 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 clear differentiation between a spectral linewidth indicative of normal operation and a spectral linewidth indicative of a fault in the spectral broadening apparatus.

[0016] In an embodiment, the band-stop optical filter is a fibre Bragg grating, FBG, used in transmission, the FBG having a reflection bandwidth configured to reflect wavelengths including the seed optical spectrum.

[0017] In an embodiment, the FBG has a reflection bandwidth corresponding to the seed optical spectrum. This may enable the FBG 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.

[0018] In an embodiment, the FBG has a reflection bandwidth corresponding to 10% to 100% of the 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 the broadened laser signal to be detected before it becomes large enough to cause unwanted effects or damage in a downstream optical device.

[0019] In an embodiment, the FBG has a reflection bandwidth 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.

[0020] In an embodiment, the FBG 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 the broadened optical spectrum 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 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 the broadened optical spectrum.

[0021] In another embodiment, the band-stop optical filter is a long-period fibre grating, LPG, used in transmission, the LPG having wavelength dependent loss at wavelengths corresponding to the seed optical spectrum.

[0022] In an embodiment, the optical filter apparatus comprises a reflective optical filter and an optical circulator. The reflective optical filter has a reflection spectrum having a reflection peak offset from a peak of the seed optical spectrum. The circulator is configured to route the optical tap signal to the reflective optical filter and to route the optical monitoring signal from the reflective optical filter to the photodetector.

[0023] In an embodiment, the reflective optical filter is a fibre Bragg grating, FBG, used in reflection, the FBG having a reflection peak offset from a peak of the seed optical spectrum. This may enable improved discrimination between a spectral linewidth indicative normal operation and a spectral linewidth indicative of a fault in the spectral broadening apparatus.

[0024] In an embodiment, the band-stop optical filter comprises an FBG used in reflection and an optical circulator. The FBG includes a central phase shift such that the FBG is configured to transmit wavelengths corresponding to the seed optical spectrum and to reflect the other wavelengths of a broadened laser signal, the reflected wavelengths forming the optical monitoring signal. The optical circulator is configured to route the optical tap signal to the FBG and to route the optical monitoring signal from the FBG to the photodetector.

[0025] In an embodiment, the seed laser is a fibre laser.

[0026] In an embodiment, the seed laser is a distributed feedback, DFB, fibre laser.

[0027] Embodiments of the disclosure will now be described, by way of example only, with reference to the accompanying drawings.

[0028] Brief Description of the drawings

[0029] Figures 1 to 9 are schematic illustrations of optical sources according to embodiments.

[0030] Detailed description

[0031] The same reference numbers are used for corresponding features in different embodiments. The present disclosure relates to an optical source including a system for monitoring the linewidth of a laser signal downstream of a spectral broadening module of the optical source. In particular, the present disclosure may provide a simple, low cost, and rugged method to monitor the performance of the spectral broadening module of an optical source, in a small and lightweight package. This may allow for the protection of high power amplifiers from damage by preventing them from being enabled, or by preventing insufficiently or unbroadened light being delivered to them, in the event of a fault with the broadening module.

[0032] More specifically, the system for monitoring the linewidth may involve tapping off a small fraction of the broadened seed laser and sending it through a narrow bandwidth filter, such as a fibre Bragg grating, FBG, or a long period grating, LPG. The power either transmitted or reflected by the filter may then be monitored using a photodiode as a measure of the linewidth of the light received from the spectral broadening module. This may offer a solution that is more efficient with a faster response, and more cost-effective, compared to alternative methods of measuring or monitoring laser linewidth using interferometers, which can be relatively complicated, expensive, and susceptible to misalignment or sensitivity to environmental conditions.

[0033] In effect, the present disclosure may provide a more efficient and cost-effective optical source including a system for protecting high power amplifiers from damage due to faults in the spectral broadening module of the optical source. This may result in a more reliable and durable high power laser source.

[0034] Referring to Figure 1 , an embodiment provides an optical source 100 comprising a seed laser 102, spectral broadening apparatus 104, an optical tap 106 and optical monitoring apparatus 120.

[0035] The seed laser 102 is configured for generating a seed laser signal having a seed laser power and a seed optical spectrum having a seed spectral linewidth. The spectral broadening apparatus 104 is configured to broaden the seed laser signal to output a broadened laser signal. The broadened laser signal has a broadened optical spectrum having a second spectral linewidth, broader than the seed spectral linewidth.

[0036] The optical tap 106 is provided downstream of the spectral broadening apparatus for forming an optical tap signal. A majority of the broadened laser signal is transmitted towards an output and a portion is split off by the optical tap to form the optical tap signal.

[0037] The optical monitoring apparatus 120 comprises an optical filter apparatus 108, a photodetector 110 and control circuitry 112. The optical filter apparatus is configured to receive the optical tap signal. The optical filter apparatus has a transmission profile configured to modify an optical power of the optical tap signal based on a spectral linewidth of the optical tap signal. Transmission of the optical tap signal through the optical filter apparatus modifies the optical tap signal, forming it into an optical monitoring signal. The photodetector 110 is configured to detect the optical monitoring signal and to output a detection signal indicative of the optical power of the monitoring optical signal. The control circuitry 112 is configured to receive the detection signal. The control circuitry is configured to output an 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.

[0038] During normal, fault free operation, the seed laser 122 generates a seed laser signal and the spectral broadening apparatus 104 broadens the seed laser signal to output a broadened laser signal having a broadened optical spectrum having a second spectral linewidth, broader than the seed spectral linewidth. A broadened laser signal is therefore output from the optical source. The seed laser signal optical spectrum is illustrated in inset (a), and the broadened laser signal optical spectrum is illustrated in inset (b).

[0039] If a fault occurs in the spectral broadening apparatus 104 which results in insufficient broadening of the seed laser signal, this will be detected by the optical monitoring apparatus 120 and an alarm signal will be output. A fault in the spectral broadening apparatus may result in the seed laser signal still being broadened but the amount of broadening not being sufficient to allow the broadened laser signal to be delivered to a downstream optical device, such as a high power fibre amplifier. A fault in the spectral broadening apparatus may alternatively result in the seed laser signal not being broadened at all, i.e. the seed laser signal is simply passed through the spectral broadening apparatus. It may be undesirable to deliver an unbroadened seed laser signal to a downstream optical device, therefore it is advantageous to output an alarm signal so that action may be taken to protect downstream optical devices receiving light from the optical source from damage. For example, an unbroadened seed laser signal should preferably not be delivered to a high power fibre amplifier, since it may result in SBS within the optical fibre amplifier and catastrophic damage to the fibre or upstream optical components.

[0040] In an embodiment, the control circuitry 112 is configured to compare the detection signal to a threshold power level and output an alarm signal if the detection signal has a power level lower than the threshold power level.

[0041] In an embodiment, the threshold power level is set at a power level above a noise level of the photodetector 110 and below a detection signal power level indicative of a second spectral linewidth of the broadened optical spectrum that is indicative of normal operation of the spectral broadening apparatus 104. The threshold power level may be set at a power level that additionally takes into account any expected optical monitoring signal variation due to environmental conditions.

[0042] In an embodiment, the photodetector 110 is a low-noise photodetector. The threshold power level is in a range 80% to 100% of a detection signal power level indicative of a second spectral linewidth that the broadened optical spectrum of the broadened laser signal should have during normal operation of the spectral broadening apparatus 104. Figure 2 illustrates an optical source 200 according to an embodiment in which the spectral broadening apparatus 210 comprises a radio frequency, RF, drive signal apparatus 202 and a phase modulator 204. The RF drive signal apparatus is configured to generate an RF drive signal having an RF signal power.

[0043] The phase modulator 204 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 second spectral linewidth. The phase modulator 204 is additionally configured to output the seed laser signal unbroadened when an RF drive signal is not received from the RF drive signal apparatus.

[0044] The optical monitoring apparatus 220 comprises an optical filter apparatus 108, a photodetector 110 and control circuitry 212. The control circuitry 212 is configured to output an alarm signal in response to the optical monitoring signal having an optical power that is indicative that the optical monitoring signal has a spectral linewidth that is indicative of a fault in the RF drive signal apparatus or the phase modulator, or both.

[0045] During normal, fault free operation, the seed laser 102 generates a seed laser signal and the phase modulator 204, driven by the RF drive signal, broadens the seed laser signal to output a broadened laser signal having a broadened optical spectrum having a second spectral linewidth, broader than the seed spectral linewidth. A broadened laser signal is therefore output from the optical source 200.

[0046] If a fault occurs in either the RF drive signal apparatus 202 or the phase modulator 204, insufficient broadening of the seed laser signal may be produced. This will be detected by the optical monitoring apparatus 220 and an alarm signal will be output. A fault in the RF drive signal apparatus may result insufficient RF drive signal power being delivered to the phase modulator. This may result in the seed laser signal still being broadened but the amount of broadening not being sufficient to allow the broadened laser signal to be delivered to a downstream optical device, such as a high power fibre amplifier.

[0047] A fault in the RF drive signal apparatus 202 may alternatively result in no RF drive signal being provided to the phase modulator 204, resulting in the seed laser signal not being broadened at all, i.e. the seed laser signal is simply passed through the spectral broadening apparatus. Alternatively, a fault in the phase modulator 204 may result in the seed laser signal not being broadened at all.

[0048] It may be undesirable to deliver an unbroadened seed laser signal to a downstream optical device, therefore it is advantageous to output an alarm signal so that action may be taken to protect downstream optical devices receiving light from the optical source from damage. For example, an unbroadened seed laser signal should preferably not be delivered to a high power fibre amplifier, since it may result in SBS within the optical fibre amplifier and catastrophic damage to the fibre. In an embodiment, the control circuitry 212 is configured to compare the detection signal to a threshold power level and output an alarm signal if the detection signal has a power level lower than the threshold power level.

[0049] In an embodiment, the threshold power level is set at a power level above a noise level of the photodetector 110 and below a detection signal power level indicative of a second spectral linewidth of the broadened optical spectrum that is indicative of normal operation of the spectral broadening apparatus 104. The threshold power level may be set at a power level that additionally takes into account any expected optical monitoring signal variation due to environmental conditions.

[0050] In an embodiment, the photodetector 110 is a low-noise photodetector. The threshold power level is in a range 80% to 100% of the detection signal power level indicative of a spectral linewidth that the broadened laser signal should have during normal operation of the spectral broadening apparatus. It will be appreciated that the closer the threshold power is to 100% of this detection signal power level the lower the photodetector noise needs to be.

[0051] Figures 3 and 4 illustrate an optical source 300 according to an embodiment in which the optical monitoring apparatus 320 comprises an optical filter apparatus 308, a photodetector 110 and control circuitry 212.

[0052] The optical filter apparatus 308 comprises a band-stop optical filter. The band-stop optical filter has an optical stop band configured to reject wavelengths corresponding to the seed optical spectrum.

[0053] The seed laser 102 generates a seed laser signal having a seed laser power and a seed optical spectrum having a seed spectral linewidth, illustrated in Figure 3 inset (a).

[0054] In this embodiment, the band-stop optical filter is a fibre Bragg grating, FBG, 308 used in transmission. The FBG has a reflection bandwidth configured to reflect wavelengths corresponding to, i.e. matching, the seed optical spectrum. The FBG reflection bandwidth is narrower than the broadened optical spectrum during normal operation of the spectral broadening apparatus. The transmission profile of the FBG is illustrated in Figure 3 inset (c)During normal, fault free operation, illustrated in Figure 3, the seed laser 102 generates a seed laser signal having a seed optical spectrum, illustrated in inset (a). The phase modulator 204, driven by the RF drive signal, broadens the seed laser signal to output a broadened laser signal having a broadened optical spectrum having a second spectral linewidth, broader than the seed spectral linewidth; the broadened optical spectrum of the broadened laser signal is illustrated in Figure 3 inset (b).

[0055] Transmission of the optical tap signal through the FBG modifies the optical tap signal, by removing (reflecting) light at wavelengths corresponding to the seed optical spectrum, to form it into an optical monitoring signal. The optical monitoring signal therefore has an optical spectrum, illustrated in Figure 3 inset (d), which is a composite of the broadened optical spectrum and the FBG transmission profile, i.e. the broadened optical spectrum with a notch corresponding to the FBG transmission profile.

[0056] If a fault occurs in either the RF drive signal apparatus 202 or the phase modulator 204, insufficient broadening of the seed laser signal may be produced. A fault in the RF drive signal apparatus may result in insufficient RF drive signal power being delivered to the phase modulator. Alternatively, a fault in the phase modulator may result in reduced broadening of the seed laser signal. Each may result in the seed laser signal still being broadened but the amount of broadening not being sufficient to allow the broadened laser signal to be delivered to a downstream optical device, such as a high power fibre amplifier. These types of fault can happen slowly over time, with the amount of broadening slowly reducing to an amount that is insufficient, and may be considered a ‘slow fail’.

[0057] A fault in the RF drive signal apparatus 202 may alternatively result in no RF drive signal being provided to the phase modulator 204, resulting in the seed laser signal not being broadened at all, i.e. the seed laser signal is simply passed through the spectral broadening apparatus. Alternatively, a fault in the phase modulator 204 may result in the seed laser signal not being broadened at all. These types of fault occur over a short period of time and may be considered a ‘fast fail’.

[0058] As illustrated in Figure 4, inset (d), if the seed laser signal is not broadened at all the optical monitoring signal will consist of only a small percentage of the unbroadened seed laser signal and will have an optical power that is indicative that the optical monitoring signal has a spectral linewidth that is indicative of a fault in the RF drive signal apparatus or the phase modulator, or both.

[0059] In an embodiment, the control circuitry 212 is configured to compare the detection signal to a threshold power level and output an alarm signal if the detection signal has a power level lower than the threshold power level.

[0060] In an embodiment, the threshold power level is set at a power level above a noise level of the photodetector 110 and below a detection signal power level indicative of a spectral linewidth of the broadened optical spectrum indicative of normal operation of the spectral broadening apparatus 104. The threshold power level may be set at a power level that additionally takes into account any expected optical monitoring signal variation due to environmental conditions.

[0061] The threshold power level is indicative that the seed laser signal has not been broadened, i.e. that there is a fault in the RF drive signal apparatus or the phase modulator, or both.

[0062] Figure 4A shows experimentally measured transmission and reflection spectra of an apodised FBG suitable for use in transmission as the FBG 308 in Figures 3 and 4. This FBG has an apodisation function that gives it a substantially rectangular reflection band having a reflection bandwidth of 25.4 pm and a substantially flat response outside the reflection band. The measured maximum transmitted optical power of this FBG was 750 |j.W and the minimum was 18 |j.W, giving a maximum contrast ratio of 16.1 dB, i.e. a good contrast between optical monitoring signal levels for a broadened laser signal and an unbroadened seed laser signal.

[0063] For a typical broadened laser signal having a broadened optical spectrum having a second spectral linewidth in the range 150 pm to 200 pm during normal operation, the FBG illustrated in Figure 4A has a reflection bandwidth corresponding to 12.7% to 16.9% of the FWHM of the broadened optical spectrum.

[0064] Figure 4B shows experimentally measured transmission and reflection spectra of another apodised FBG suitable for use in transmission as the FBG 308 in Figures 3 and 4. This FBG has an apodisation function that gives it a substantially rectangular reflection band having a reflection bandwidth of 33.6 pm and a substantially flat response outside the reflection band. The measured maximum transmitted optical power of this FBG was 650 |j.W and the minimum was 4.5 |j.W, giving a maximum contrast ratio of 21 .6 dB.

[0065] For a typical broadened laser signal having broadened optical spectrum having a spectral linewidth in the range 150 pm to 200 pm during normal operation, the FBG illustrated in Figure 4B has reflection bandwidth corresponding to 16.8% to 22.4% of the FWHM of the broadened optical spectrum.

[0066] Figure 5 illustrates an optical source 350 according to an embodiment in which the optical monitoring apparatus 360 comprises an optical filter apparatus 308, a photodetector 110 and control circuitry 312.

[0067] The control circuitry 312 is configured to compare the detection signal to two threshold power levels and output an alarm signal if the detection signal has a power level lower than one or both threshold power levels. The threshold power levels may be set well above a noise level of the photodetector 110 and that take into account any expected optical monitoring signal variation due to environmental conditions.

[0068] A first threshold power level is indicative of the seed laser signal having a normal optical power level. A second, lower threshold power level is indicative of no spectral broadening.

[0069] The control circuitry 312 is configured to compare the detection signal to the first threshold power level and to the second threshold power level. If the detection signal has a power level lower than the first threshold power level, but higher than the second threshold power level, this is indicative of a drop in optical power of the seed laser. The control circuitry then outputs an alarm signal indicative of a fault in the seed laser. If the detection signal has a power level lower than both the first threshold power level and the second threshold power level, this is indicative of no spectral broadening. The control circuitry then outputs an alarm signal indicative that no spectral broadening has occurred. Figure 6 illustrates an optical source 400 according to an embodiment in which the optical monitoring apparatus 420 comprises an optical filter apparatus 408, a photodetector 110 and control circuitry 312.

[0070] The optical filter apparatus 408 comprises a band-stop optical filter. The band-stop optical filter has an optical stop band configured to reject wavelengths corresponding to the seed optical spectrum.

[0071] In this embodiment, the band-stop optical filter is a long period grating, LPG, 408. The LPG has wavelength dependent loss at wavelengths corresponding to the seed optical spectrum. The transmission profile of the LPG is illustrated in Figure 6 inset (c).

[0072] Figures 7 and 8 illustrate an optical source 500 according to an embodiment in which the optical monitoring apparatus 520 comprises an optical filter apparatus 508, a photodetector 110 and control circuitry 312. The optical filter apparatus 508 comprises a reflective optical filter 502 and an optical circulator 504.

[0073] The reflective optical filter has a reflection spectrum having a reflection peak offset from a peak of the seed optical spectrum. In this embodiment, the reflective optical filter is a fibre Bragg grating, FBG, 508 having a reflection peak offset from a peak of the seed laser signal optical spectrum. The seed laser signal optical spectrum is illustrated in inset (a), the broadened optical spectrum is illustrated in inset (b) and the reflection spectrum of the FBG is illustrated in inset (c). It will be appreciated by the skilled person that the peak of the broadened optical spectrum is at substantially the same wavelength as the peak of the seed laser signal optical spectrum.

[0074] The circulator 504 is configured to route the optical tap signal to the FBG and to route the optical monitoring signal from the FBG to the photodetector 110.

[0075] During normal operation, i.e. when the seed optical signal is broadened by the PM 204, as illustrated in Figure 7, reflection of the broadened laser signal by the FBG 502 will form an optical monitoring signal having two peaks, as shown in inset (d). The large peak corresponds to light within the broadened optical spectrum at wavelengths corresponding to the peak of the FBG reflection spectrum and the small peak corresponds to light within the broadened optical spectrum at wavelengths at the peak of the broadened optical spectrum, which is at wavelengths in the low wavelength tail of the FBG reflection spectrum.

[0076] As illustrated in Figure 8, if there is a fault in the PM 204 or the RF drive signal apparatus 202 resulting in no broadening of the seed laser signal, reflection of the unbroadened seed laser signal by the FBG 502 will form an optical monitoring signal having a single, low peak, as shown in inset (d). Because the peak of the FBG reflection spectrum is offset from the peak of the seed laser signal optical spectrum, the entire seed laser optical spectrum sits in the low wavelength tail of the FBG reflection spectrum, i.e. at wavelengths where reflection by the FBG is low. Figure 9 illustrates an optical source 600 according to an embodiment in which the optical monitoring apparatus 520 comprises a band-stop optical filter 608, a photodetector 110 and control circuitry 312.

[0077] The band-stop optical filter 608 comprises an FBG 602 and an optical circulator 504. The FBG includes a central phase shift. This means that the FBG has a reflection spectrum which has a central band-pass notch, shown in inset (c). The band-pass notch covers wavelengths corresponding to the seed optical spectrum. The FBG is thus configured to transmit wavelengths corresponding to the seed optical spectrum and to reflect the other wavelengths within the broadened optical spectrum of a broadened laser signal. The reflected wavelengths form the optical monitoring signal, which has a central notch corresponding to the band-pass notch of the FBG, as shown in inset (d).

[0078] The optical circulator 504 is configured to route the optical tap signal to the FBG 602 and to route the optical monitoring signal from the FBG 602 to the photodetector 110.

[0079] Figure 9A shows experimentally measured transmission and reflection spectra of an FBG with a central phase shift suitable for use in reflection as the FBG 602 in Figure 9. The FBG has a TI phase shift that gives it a sharp transmission peak at the centre of the spectrum; the transmission peak has a FWHM linewidth of 2.6 pm and the reflection is substantially flat outside this region for greater than + / - 100 pm from the centre, which is wide enough to accommodate the whole of the spectrum of the broadened laser signal. The measured maximum reflected optical power of this FBG was 430 |j.W and the minimum was 1 .7 |j.W, giving a maximum contrast ratio of 24.0 dB, i.e. a good contrast between optical monitoring signal levels for a broadened laser signal and an unbroadened seed laser signal.

[0080] Referring to Figures 10 and 10A, an embodiment provides an optical source 700, similar to the embodiment illustrated in Figures 3 and 4, in which the optical monitoring apparatus 720 comprises an optical filter apparatus 708, a photodetector 110 and control circuitry 212.

[0081] The optical filter apparatus 708 comprises an FBG used in transmission. The FBG 708 has a reflection bandwidth corresponding to 10% to 100% of the FWHM of the broadened optical spectrum of the broadened laser signal for normal operation of the spectral broadening apparatus.

[0082] Figures 10A and 10B illustrate modelling of transmission of a broadened laser signal having a Gaussian broadened optical spectrum having a second spectral linewidth in the range 50 pm to 140 pm through an FBG 708 having a reflection bandwidth in the range 20 pm to 50 pm. These figures illustrate that the sensitivity of the optical monitoring apparatus 720 may be increased by increasing the width of the FBG reflection bandwidth relative to the second spectral linewidth of the broadened optical spectrum.

[0083] Figure 10A models transmission of a broadened laser signal having a Gaussian broadened optical spectrum having a FWHM second spectral linewidth in the range 50 pm to 130 pm through an FBG 708 having a reflection bandwidth of 25 pm (equivalent to 19.2% - 50% of the FWHM of the broadened optical spectrum).

[0084] Figure 10B models transmission of a broadened laser signal having a Gaussian broadened optical spectrum having a FWHM second spectral linewidth of 140 pm through an FBG 708 having a reflection bandwidth in the range 20 pm to 50 pm (equivalent to 14.3% - 35.7% FWHM of the broadened optical spectrum). Figure 10B also illustrates that the sensitivity of the optical monitoring apparatus 720 to a change in the second spectral linewidth of the broadened optical spectrum improves as the FBG reflection bandwidth gets wider relative to the FWHM of the broadened optical spectrum - this is seen in the increasing slope of the curves as the reflection bandwidth increases; a wider reflection bandwidth results in a steeper response (decrease in optical monitoring signal power) to small reductions in the second spectral linewidth of the broadened optical spectrum.

[0085] In certain embodiments, the seed laser is a fibre laser, such as a distributed feedback, DFB, fibre laser.

[0086] Additional Embodiments

[0087] 1. An optical source (100, 200, 300, 350, 400, 500) comprising: a seed laser (102) for generating a seed laser signal having a seed laser power and a seed spectral linewidth; spectral broadening apparatus (104, 210) operable to broaden the seed laser signal to output a broadened laser signal having a second spectral linewidth, broader than the seed spectral linewidth; an optical tap (106) downstream of the spectral broadening apparatus for forming an optical tap signal; and optical monitoring apparatus (120, 220, 320, 360, 420, 520) comprising: optical filter apparatus (108, 308, 408, 508) configured to receive the optical tap signal, the optical filter apparatus having a transmission profile configured to form an optical monitoring signal by modifying an optical power of the optical tap signal dependent on a spectral linewidth of the optical tap signal; a photodetector (110) configured to detect the optical monitoring signal and to output a detection signal indicative of the optical power of the monitoring optical signal; and control circuitry (112, 212, 312) 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.

[0088] 2. The optical source of embodiment 1 , wherein the spectral broadening apparatus (210) comprises: radio frequency, RF, drive signal apparatus (202) operative to generate an RF drive signal having an RF signal power; and a phase modulator (204) configured to apply phase modulation to the seed laser signal when an RF drive signal is received to form a broadened laser signal having the second spectral linewidth and operative to output the seed laser signal unbroadened when an RF drive signal is not received, and wherein the control circuitry (212) 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. The optical source of embodiment 1 or embodiment 2, wherein the control circuitry (112, 212) is configured to: compare the detection signal to a threshold indicative of a spectral linewidth indicative of normal operation of the spectral broadening apparatus; and output a said alarm signal if the detection signal has a power lower than the threshold. The optical source of any one of the preceding embodiments, wherein the optical filter apparatus comprises a band-stop optical filter (308, 408, 608) having an optical stop band configured to reject wavelengths corresponding to the seed optical spectrum. The optical source of embodiment 4, wherein the band-stop optical filter is one of a fibre Bragg grating, FBG, (308) having a reflection bandwidth configured to reflect wavelengths corresponding to the seed optical spectrum or a long-period fibre grating, LPG, (408) having wavelength dependent loss at wavelengths corresponding to the seed optical spectrum. The optical source of any one of embodiments 1 to 3, wherein the optical filter apparatus (508) comprises a reflective optical filter (502) and an optical circulator (504), wherein the reflective optical filter has a reflection spectrum having a reflection peak offset from a peak of the seed optical spectrum and the circulator is configured to route the optical tap signal to the reflective optical filter and to route the optical monitoring signal from the reflective optical filter to the photodetector. The optical source of embodiment 6, wherein the reflective optical filter is a fibre Bragg grating, FBG, (508) having a reflection peak offset from a peak of the seed optical spectrum. The optical source of embodiment 4, wherein the band-stop optical filter (608) comprises: an FBG (602) including a central phase shift such that the FBG is configured to transmit wavelengths corresponding to the seed optical spectrum and to reflect the other wavelengths of a broadened laser signal; and an optical circulator (504), arranged to route the optical tap signal to the FBG and to route the optical monitoring signal from the FBG to the photodetector. The optical source of any one of the preceding embodiments, wherein the seed laser (102) is a fibre laser. The optical source of embodiment 9, wherein the seed laser (102) is a distributed feedback, DFB, fibre laser.

Claims

CLAIMS1. An optical source (100, 200, 300, 350, 400, 500) comprising: a seed laser (102) configured for generating a seed laser signal having a seed laser power and a seed optical spectrum having a seed spectral linewidth; spectral broadening apparatus (104, 210) configured to broaden the seed laser signal to output a broadened laser signal having a broadened optical spectrum having a second spectral linewidth, broader than the seed spectral linewidth; an optical tap (106) downstream of the spectral broadening apparatus for forming an optical tap signal; and optical monitoring apparatus (120, 220, 320, 360, 420, 520) comprising: optical filter apparatus (108, 308, 408, 508) configured to receive the optical tap signal, the optical filter apparatus having a transmission profile configured to form an optical monitoring signal by modifying an optical power of the optical tap signal based on a spectral linewidth of the optical tap signal; a photodetector (110) configured to detect the optical monitoring signal and to output a detection signal indicative of the optical power of the monitoring optical signal; and control circuitry (112, 212, 312) 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.

2. The optical source of claim 1 , wherein the spectral broadening apparatus (210) comprises: radio frequency, RF, drive signal apparatus (202) configured to generate an RF drive signal having an RF signal power; and a phase modulator (204) configured to apply phase modulation to the seed laser signal when an RF drive signal is received to form a broadened laser signal having the second spectral linewidth and configured to output the seed laser signal unbroadened when an RF drive signal is not received, and wherein the control circuitry (212) 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.

3. The optical source of claim 1 or claim 2, wherein the control circuitry (112, 212) is configured to: compare a power of the detection signal to a threshold power level; andoutput a said alarm signal if the detection signal has a power level lower than the threshold power level.

4. The optical source of claim 3, wherein the threshold power level is set at a power level above a noise level of the photodetector and below a detection signal power level indicative of a second spectral linewidth of the broadened optical spectrum indicative of normal operation of the spectral broadening apparatus.

5. The optical source of claim 3, wherein the photodetector is a low-noise photodetector and the threshold power level is in a range 80% to 100% of a detection signal power level indicative of a spectral linewidth indicative of normal operation of the spectral broadening apparatus.

6. The optical source of any one of the preceding claims, wherein the optical filter apparatus comprises a band-stop optical filter (308, 408, 608) having an optical stop band configured to reject wavelengths corresponding to the seed optical spectrum.

7. The optical source of claim 6, wherein the band-stop optical filter is a fibre Bragg grating, FBG, (308) used in transmission, the FBG having a reflection bandwidth configured to reflect wavelengths including the seed optical spectrum.

8. The optical source of claim 7, wherein the FBG has a reflection bandwidth corresponding to the seed optical spectrum.

9. The optical source of claim 7, wherein the FBG has a reflection bandwidth 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.

10. The optical source of claim 9, wherein the FBG has a reflection bandwidth corresponding to 10% to 30% of the FWHM of the broadened optical spectrum for normal operation of the spectral broadening apparatus.

11. The optical source of claim 9, wherein the FBG 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.

12. The optical source of claim 6, wherein the band-stop optical filter is a long-period fibre grating, LPG, (408) used in transmission, the LPG having wavelength dependent loss at wavelengths corresponding to the seed optical spectrum.

13. The optical source of any one of claims 1 to 3, wherein the optical filter apparatus (508) comprises a reflective optical filter (502) and an optical circulator (504), wherein the reflective optical filter has a reflection spectrum having a reflection peak offset from a peak of the seed optical spectrum and wherein the circulator is configured to route the optical tap signal to the reflective optical filter and to route the optical monitoring signal from the reflective optical filter to the photodetector.

14. The optical source of claim 13, wherein the reflective optical filter is a fibre Bragg grating, FBG, (508) used in reflection, the FBG having a reflection peak offset from a peak of the seed optical spectrum.

15. The optical source of claim 6, wherein the band-stop optical filter (608) comprises: an FBG (602) used in reflection, the FBG including a central phase shift such that the FBG is configured to transmit wavelengths corresponding to the seed optical spectrum and to reflect the other wavelengths of a broadened laser signal, the reflected wavelengths forming the optical monitoring signal; and an optical circulator (504), configured to route the optical tap signal to the FBG and to route the optical monitoring signal from the FBG to the photodetector.

16. The optical source of any one of the preceding claims, wherein the seed laser (102) is a fibre laser.

17. The optical source of claim 16, wherein the seed laser (102) is a distributed feedback, DFB, fibre laser.

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