Delayed Self Heterodyne Detections Biology Essay

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The linewidth feature of a commercial tunable optical maser beginning is by experimentation demonstrated with two different linewidth measuring methods ; homodyne and heterodyne sensings.

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With 25km of SMF as a hold fibre and broad manner operation, the linewidths of 124 MHz, 114 MHz and 176 MHz are obtained at runing wavelengths of 1530 nanometer, 1540 nanometer and 1560 nanometer, severally. The linewidth of the TLS is obtained at around 10 MHz and 150 MHz for narrow and broad manners, severally with heterodyne technique.

Index Footings: optical maser linewidth measuring, tunable optical maser beginning ( TLS ) , homodyne and heterodyne sensings. SMF

I- INTRODUCTION

Recently developed optical maser beginnings show improved coherency belongingss and linewidth in the sub-megahertz scope will be available in a close hereafter even in massive semiconducting material optical masers. The linewidth measuring of such optical masers is a new challenge, since classical self-homodyne or self-heterodyne techniques require inordinate lengths of detaining fiber and are therefore non operable [ 1,2 ] .

The linewidth of a optical maser is the full breadth at half-maximum ( FWHM ) of its optical spectrum.

The linewidth is strongly related to the temporal coherency, characterized by the coherency clip or coherency length. A optical maser linewidth can be measured with a assortment of techniques such as utilizing optical spectral analysis every bit good as change overing the frequence fluctuation into strength fluctuation. For single-frequency optical masers, the self-heterodyne technique is frequently used, which involves entering a all in note between the optical maser end product and a frequency-shifted and delayed version of it. For sub-kilohertz linewidths, the ordinary self-heterodyne technique normally becomes impractical, but it can be extended by utilizing a recirculating fibre cringle with an internal fibre amplifier.

Another technique is based on self-heterodyne sensing induced by Stimulated Brillouin Scattering ( SBS ) to present a jitter-insensitive sub-KHz declaration linewidth word picture technique in ultra-narrow optical masers for optical communicating applications [ 3 ] . Lasers with really narrow linewidth are required for assorted applications such as light beginnings for assorted sorts of fiberoptic detectors, for LIDAR spectrometry, in consistent optical fibre communications, and for trial and measuring intents [ 4 ] .

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1Dr Adnan H. Ali is presently a Ph D. grade in Laser and Opto-electronic Engineering from university of Technology-Baghdad, He is now with Mechatronics Technique Department at Technical college - Baghdad. ( adnan_h_ali @ yahoo.com ) .

2Dr Sahib N. Abdul-Wahid is presently a Ph D. grade in Laser Physics from university of Baghdad, He is now Dean deputy of Basic instruction college at Kufa university. ( al_gharany @ yahoo.com )

The linewidth of a optical maser depends strongly on the type of optical maser. It may be farther minimized by optimising the optical maser design and stamp downing external noise influences every bit far as possible. The first measure should be to find whether self-generated emanation noise is ruling, because the needed steps can depend really much on this. The influence of self-generated emanation noise is little for a optical maser with high intra-cavity power, low resonating chamber losingss, and a long resonating chamber round-trip clip. Single frequence fibre optical masers can accomplish linewidths of a few kHz, or sometimes even below 1A kilohertz. The linewidth of a optical maser rectifying tube is typically in the megahertz part, but it can besides be reduced to a few kHz, e.g. in external-cavity rectifying tube optical masers, peculiarly with optical feedback from a high-finesse mention pit [ 5 ] . Small fibre optical masers in the signifier of distributed feedback optical masers ( with the resonating chamber formed basically by a particular fibre Bragg grating ) can bring forth 10s of millwatts of end product power with a linewidth in the part of a few kHz.

The Brillouin addition in optical fibres can be used to do narrow linewidth optical masers by puting a piece of extremely nonlinear fibre inside a pit. We can utilize the nonlinear Brillouin addition to bring forth Brillouin fiber optical maser, which can be generated at any wavelength depending on the handiness of the Brillouin pump.

II- LINEWIDTH MEASUREMENT TECHNIQUES

A. Self-homodyne sensing

In consistent sensing, the frequence of the input optical signal is down converted to the RF sphere through blending with a local oscillator. Self-homodyne sensing eliminates the demand of a local oscillator, and the optical signal mixes with a delayed version of itself. Fig.1 shows the self-homodyne sensing set-up, where two hold lines are used between two directional couplings to organize a Mach-Zehnder constellation [ 6 ] .

B. Heterodyne Detection

Heterodyne sensing can supply non merely laser linewidth informations, but besides optical power spectrum. This method is the lone technique that is capable of qualifying non symmetrical spectral lineshape. This method besides offers high sensitiveness and high declaration. The cardinal constituent required for this method is a stable, narrow line-width mention optical maser. In heterodyne sensing, two optical masers have to be used. One is the signal optical maser and another 1 is a mention optical maser, which can be referred as the local oscillator ( LO ) . The cardinal frequence of the LO optical maser must be tuned near to the signal optical maser frequence to let the blending merchandise to fall within the bandwidth of sensing device.

Fig.2 shows the typical experimental set-up for heterodyne sensing method whereby a visible radiation from the LO is combined with the signal optical maser under trial by a 3 dubnium coupling. The coupling combines the two Fieldss, presenting partial power to each end product port. One port leads to a photodetector ( PD ) which detects the intervention round note, change overing it to an electrical tone.

Fig. 1: Typical experimental set-up for linewidth measuring utilizing self-homodyne method.

Fig.2 Typical experimental set-up for linewidth measuring utilizing a heterodyne round technique.

The chief trouble to utilize heterodyne sensing is that two optical masers must be used and the linewidth of the mention optical maser must be narrower than or at least comparable to that of the optical maser beginning to be measured in order to accomplish sensible measuring truth [ 7 ] .

C. Delayed self-heterodyne sensing

Compared with heterodyne sensing, delayed self-heterodyne sensing provides a simpler method to execute optical maser linewidth measuring without utilizing a separate local oscillator. Alternatively, delayed self-heterodyne sensing needs a big optical hold. The delayed self-heterodyne interferometer ( DSHI ) construct is shown in Fig. 3. Incident visible radiation is split into two waies by the interferometer. The optical frequence of one arm is offset with regard to the other. If the hold, I„d, of one way exceeds the coherency clip, I„c of the beginning, the two uniting beams interfere as if they originated from two independent optical masers offset in frequence by I?I? . Thus the system performs likewise to optical heterodyne sensing.

III- SELF- HOMODYNE LINEWIDTH MEASUREMENT

A linewidth measuring for a commercial tunable optical maser beginning ( TLS ) is demonstrated in this chapter. Fig. 4 shows the experimental set-up, which consists of TLS ( ANDO AQ4321D ) , 1 x 2 3dB coupling, a piece of long SMF ( 25km & A ; 50km ) length, photo-detector, and 8GHz wireless frequence spectrum analyser. The input visible radiation from TLS is split by the first 3 dubnium coupling, which one arm is connected to the 2nd 3 dubnium coupling. Another arm of the coupling is connected to a piece of hold SMF connected to the 2nd 3 dubnium coupling. The combined visible radiation at 2nd coupling is converted into electrical signal by a photodiode, which the signal is analyzed by the RFSA.

Fig. 5 shows the end product spectrum of TLS at power scene of 0 dBm, linewidth scene of 'wide ' and wavelength of 1550 nanometer. Fig. 6 shows the all in frequence spectrum at three different runing wavelengths. In the experiment, the TLS power and SMF length is fixed at 0 dBm and 25 kilometer, severally. The linewidths of 124 MHz, 114 MHz and 176 MHz are obtained at runing wavelengths of 1530 nanometer, 1540 nanometer and 1560 nanometer, severally. The values of the linewidth are obtained at 3 dubnium from the peak signal from the graphs in Fig. 6. Fig 7 shows the all in frequence spectrum at three different runing wavelengths but utilizing a longer delay fibre of 50 kilometer. The linewidths of 320 MHz, 120 MHz and 194 MHz are obtained at runing wavelengths of 1530 nanometer, 1540 nanometer and 1560 nanometer, severally. It is found that a somewhat higher value is obtained with a longer length of the hold fibre.

Fig. 8 shows the linewidth of visible radiation against wavelength of the TLS at two different hold fibres. In the experiment, the TLS option is set as 'wide ' manner. As shown in the figure, the mean linewidth of the TLS is about 200 MHz. Since the consequence for 25 kilometers and 50 kilometers hold fibres are about similar, it is concluded that 25 kilometers long of SMF is adequate of provide hold for the self-homodyne linewidth

Fig. 3: Conventional apparatus for optical delayed self-heterodyne sensing

Fig. 4: Experimental set-up for self-homodyne linewidth measuring

Fig. 5: The spectrum of the input signal from TLS

Fig. 6: The all in signal curves against all in frequence at different wavelengths which were obtained utilizing 25 kilometer

Fig. 7: The all in signal curves against all in frequence at different wavelengths which were obtained utilizing 50 kilometer

Fig 8: Relationship between linewidth and operating wavelength

IV- HETERODYNE TECHNIQUE FOR LINEWIDTH MEASUREMENT

The ultra-narrow linewidth measuring is a challenge because the self- homodyne or self-heterodyne techniques require long delaying fibres which is impractical for fibre optical masers due to extension losingss. The standard spectrum analyser besides has a really big dark country of more than 20MHz, which is impractical to mensurate a really narrow linewidth optical maser. However, the optical maser linewidth measuring can besides be done by measuring the all in signal ensuing from the intervention of the optical maser with another uncorrelated optical maser [ 8 ] . This measuring technique is known as heterodyne method. This method requires another optical maser either with a comparable well-known spectrum or with an highly narrow and ignorable linewidth. Fig. 9 shows the experimental set, which consists of two optical maser beginnings ( TLS 1 and TLS 2 ) , two isolators, a Personal computer, a 3 dubnium coupling, sensor and RFSA. Optical isolator is used to avoid the backward contemplation from damaging the TLS. Personal computer is used to set the polarisation province of one of the light beginning.

Fig. 10 shows the all in signal from the RF spectrum analyser when both TLS scenes are set as narrow manner. In the experiment, the input signal power and wavelength for both TLSs are set at 0 dBm and 1550 nanometer, severally. Many all in frequence signals are observed due to the fluctuation in the TLS 's wavelength. The linewidth of the light beginning is estimated to be about 6 MHz as shown in the inset of Fig. 10. Fig. 11 shows the linewidth spectrum of the trial signal ( TLS 2 ) at two different linewidth scenes. In the experiment, the mention beginning ( TLS 1 ) is set with narrow manner. Fig. 11 ( a ) and ( B ) shows the spectrum when the TLS 2 is set with narrow and broad manners, severally. As shown in both figures, the linewidth of 7 MHz and 58 MHz are obtained with the narrow and broad manners, severally.

Fig. 12 summarizes the relationship between the linewidth value and the signal wavelength for two different manners of TLS 2. As shown in the figure, the linewidth value is smaller at the longer wavelength for both mode scenes. In norm, the linewidth of the TLS is obtained at around 10 MHz and 150 MHz for narrow and broad manners, severally as shown in Fig. 12. Compared to the old homodyne technique, the linewidth value obtained for broad manner is about the same. This proves the accurac0y of this technique.

5- CONCLUSION

Lasers with really narrow linewidth are required for assorted applications. The homodyne and heterodyne measuring techniques are the most constituted method for mensurating the linewidth of a optical maser. In the homodyne measuring, With 25km of SMF as a hold fibre, the linewidths of 124 MHz, 114 MHz and 176 MHz are obtained at runing wavelengths of 1530 nanometer, 1540 nanometer and 1560 nanometer, severally. In the experiment, the TLS scene is set at 'wide manner ' and the values of the linewidth are obtained at 3 dubnium from the peak signal. At 50 kilometer SMF, the linewidths of 320 MHz, 120 MHz and 194 MHz are obtained at runing wavelengths of 1530 nanometer, 1540 nanometer and 1560 nanometer, severally. This consequence shows that a somewhat higher value is obtained with a longer length of the hold fibre. The linewidths of 180 MHz, 210 MHz and 220 MHz are obtained at runing power of ( 0 dBm ) , ( -4 dBm ) , and ( -8 dBm ) , severally. This consequence shows the linewidth value increases with the decrease of signal power.

In norm, the linewidth of the TLS is obtained at around 10 MHz and 150 MHz for narrow and broad manners, severally. The value obtained is comparable to the consequence obtained in the old homodyne technique for broad manner [ 1 ] . This proves the truth of this technique.

Fig.9: Configuration of heterodyne linewidth measuring

Fig. 10: Beat frequence signals from RFSA. Inset shows the hypertrophied image at all in frequence of 639 MHz.

( a ) Narrow manner ( B ) Wide manner

Fig. 11: Linewidth spectrum of the trial signal ( TLS 2 ) at two different linewidth scenes

Fig. 12: Linewidth against wavelength for two different manners of TLS 2.