CN105699053A - Device and method for precisely measuring laser line width on the basis of cyclic self-heterodyne interferometry - Google Patents

Abstract

The invention discloses a device and method for precisely measuring a laser line width on the basis of cyclic self-heterodyne interferometry. The device comprises a narrow line width laser which is successively connected with an isolator, a first Y-type coupler, a first frequency shifter, a second Y-type coupler, a photoelectric detector, and a frequency spectrograph via fibers. An X-type coupler is connected between the first Y-type coupler and the second Y-type coupler via fibers. The X-type coupler is successively connected with an attenuator, a second frequency shifter, an erbium-doped fiber amplifier, a fiber band-pass filter, a polarization maintainer via fibers so that a closed loop is formed. The device and the method uses cyclic self-heterodyne interferometry based on loss compensation, and a short fiber delay line, does not require complex theoretical simulation and accurate system parameter setting, and may measure laser linewidths at a sub-kHz magnitude accurately.

Description

Accurate measurement laser linewidth device and method based on circulation self-heterodyne interferometric method

Technical field

The invention belongs to spectral measurement methods field, relate to the device and method of a kind of accurate measurement laser linewidth, especially a kind of based on circulation self-heterodyne interferometric method, adopt frequency spectrum isolation technics to reach the device and method of accurate measurement laser linewidth。

Background technology

Narrow-linewidth laser, as a kind of means of high-accuracy measurement, has important in scientific research and technical field and is widely applied, such as light clock, high-accuracy spectrum, coherent communication, laser ranging etc.。The noiseproof feature of these systems, measurement distance, accuracy and sensitivity etc. are played decisive role by the live width of laser or coherence length。Therefore the live width of narrow laser instrument is carried out high-acruracy survey and seem most important。At present, the measuring method of spectral line width substantially has three kinds: spectrometer measurement method, filter scan method, beat frequency method。But when the live width of laser instrument reaches kHz magnitude, needed for traditional Measurement Resolution based on grating spectrograph and filter scan method can not meet experiment。Heterodyne beat method is a kind of measurement super-narrow line width ideal method, it is possible to obtain satisfied resolution。Wherein, the laser linewidth measurement technology based on time delay self-heterodyne method is most widely used, because it does not need an other equal even more steady laser instrument as reference light source。

In recent years, lot of domestic and international group carries out deep theoretical research and experimental exploring for self-heterodyne Laser Measurement live width。Within 1980, Japanese scholars T.Okoshi proposes time delay self-heterodyne method (DSHI) Laser Measurement device live width first, and obtains the resolution of 50kHz。In this kind of scheme, the length of fiber delay line is more than more than six times of laser coherence length, and beat signal could correctly reflect the live width of measured laser。Therefore for the narrow linewidth laser of a large amount of kHz magnitudes used at present, fibre delay line required for Laser Measurement live width is likely to need kilometer up to a hundred, this makes whole test systems bulky, and additional noise increases, and limits its application in ultra-narrow laser linewidth is measured。

1986, the L.E.Richter analytical derivation theoretically principle of time delay self-heterodyne method Laser Measurement live width, and propose to adopt short fiber time delay, the Laser Measurement device live width when fiber delay time laser coherence time much smaller than 6 times。In this kind of scheme, theoretical spectral pattern can not well match with experimental spectrum shape, and the delta function summit simultaneously embodying coherence affects the reading of actual linewidth, finally affects the accuracy of data。

1992, J.W.Dawson proposed the scheme of a kind of loop self-heterodyne interferometer (LC-RDSHI) Laser Measurement live width utilizing loss balancing。Comparing DSHI system, the laser in this scheme can repeatedly circulate in the loop, thus the length of very effective minimizing time delay optical fiber。2006, X.P.Chen et al. utilized LC-RDSHI method Laser Measurement live width first in an experiment, is finally analyzed measurement result revising, obtains laser linewidth and be about 700Hz。But this experimental program needs system parameters carries out strict selection and carefully debugging, could reduce the cross-couplings disturbing light pair in loop, thus improving the signal to noise ratio of high-order beat frequency spectrum。

Summary of the invention

It is an object of the invention to the shortcoming overcoming above-mentioned prior art, it is provided that a kind of accurate measurement laser linewidth device and method based on circulation self-heterodyne interferometric method, it has the feature of simple to operate, high s/n ratio and low cost。

It is an object of the invention to be achieved through the following technical solutions:

Present invention firstly provides a kind of accurate measurement laser linewidth device based on circulation self-heterodyne interferometric method, including narrow linewidth laser, described narrow linewidth laser is connected with isolator, a Y type bonder, the first frequency shifter, the 2nd Y type bonder, photodetector and audiofrequency spectrometer in turn by optical fiber；It is also associated with X-coupler by optical fiber between a described Y type bonder and the 2nd Y type bonder；Described X-coupler is connected formation closed circuit successively also by optical fiber with attenuator, the second frequency shifter, erbium-doped fiber amplifier, fiber band-pass filter, the inclined device of guarantor。

Further, a end of above-mentioned X-coupler is connected to the outfan of a Y type bonder, and the c end of X-coupler is connected to the input of the 2nd Y type bonder。

Further, the d end of above-mentioned X-coupler is connected to the input of attenuator, and the b end of X-coupler is connected with the outfan protecting inclined device。

The above-mentioned outfan of the second frequency shifter is connected with the input of erbium-doped fiber amplifier。

The present invention also proposes a kind of method of accurate measurement laser linewidth based on said apparatus, comprises the following steps:

Step 1: the light of measured laser device output is divided into two parts by a Y type bonder, a part, as reference light, enters the 2nd Y type bonder through the first frequency shifter；Another part, as flashlight, enters in the self-loopa light path being connected by X-coupler b, d two ends and formed；N time circulation time delayed signal light export by the c end of X-coupler, and enter the 2nd Y type bonder carry out from the reference light of different propagation paths interfere beat frequency, and pass through photodetector and audiofrequency spectrometer acquisition laser instrument optogalvanic spectra density。When the delay time of fiber optic loop is much larger than coherence time of laser, optogalvanic spectra density just can reflect the live width of laser instrument。

Step 2: utilize Voigt spectral pattern analysis, extracts the 1/f measurement error caused, revises measurement result。

The method have the advantages that

Accurate measurement laser linewidth device and method of the present invention is based on the circulation self-heterodyne interference technique of loss balancing, adopts shorter fiber delay line, it is not necessary to complicated theoretical modeling and accurately system parameter settings, can accurately measure out the laser linewidth of sub-kHz magnitude。

Further, the present invention using the output light of tested narrow linewidth laser by being become two-way by first Y type bonder light splitting after isolator: a road light as reference light, after frequency shifter frequency displacement enter second Y type bonder；Another road light enters one as flashlight and is joined end to end in the self-loopa light path constituted by X-coupler, two Y type bonders are arrived after optical fiber loop circulates m time, and at its outfan with reference to photosynthetic beam interferometer, finally interference beat signal input detector and spectrum analyzer carry out acquisition of signal and analysis。When the delay inequality of the two difference path light beam is much larger than coherence time of laser, their beat signal of interfering just can correctly reflect the live width of measured laser device。Respectively to reference light and flashlight shift frequency in optical fiber loop, and the shift frequency amount of reference light is not equal to the integral multiple of flashlight shift frequency, so without the parameter of detailed computing system, just the measurement error that separable, elimination interference optical signal causes, thus obtaining the wire width measuring value of the high narrow linewidth laser of precision。

Accompanying drawing explanation

Fig. 1 is the structural representation of laser linewidth measurement apparatus of the present invention；

Fig. 2 is the 10 rank beat signals that traditional LC-RDSHI method records；

Fig. 3 is the 10 rank beat signals that apparatus of the present invention record；

Fig. 4 is that Voigt spectral pattern is to experimental data correction result；

Wherein: 1 narrow linewidth laser, 2 isolators, 3 the oneth Y type bonders, 4 first frequency shifters, 5X type bonder, 6 attenuators, 7 second frequency shifters；8 erbium-doped fiber amplifiers；9 fiber band-pass filters；10 protect inclined device；11 the 2nd Y type bonders；12 photodetectors；13 audiofrequency spectrometers。

Detailed description of the invention

Below in conjunction with accompanying drawing, the present invention is described in further detail:

Referring to Fig. 1: present invention firstly provides a kind of accurate measurement laser linewidth dress based on circulation self-heterodyne interferometric method: including narrow linewidth laser 1, described narrow linewidth laser 1 is connected with isolator the 2, the oneth Y type bonder the 3, first frequency shifter the 4, the 2nd Y type bonder 11, photodetector 12 and audiofrequency spectrometer 13 in turn by optical fiber；It is also associated with X-coupler 5 by optical fiber between a described Y type bonder 3 and the 2nd Y type bonder 11；Described X-coupler 5 also by optical fiber successively with attenuator the 6, second frequency shifter 7, erbium-doped fiber amplifier 8, fiber band-pass filter 9, protect inclined device 10 be connected formation closed circuit。The a end of described X-coupler 5 is connected to the outfan of a Y type bonder 3, and the c end of X-coupler 5 is connected to the input of the second bonder 11。The d end of described X-coupler 5 is connected to the input of attenuator 6, and the b end of X-coupler 5 is connected with the outfan protecting inclined device 10。The described outfan of the second frequency shifter 7 is connected with the input of erbium-doped fiber amplifier 8。

Based on said apparatus, the present invention proposes a kind of method of accurate measurement laser linewidth, comprises the following steps:

Step 1: the output light of tested narrow linewidth laser is by being become two-way by first Y type bonder light splitting after isolator: a road light, as local reference light, enters second Y type bonder through the first frequency shifter (frequency shift amount Λ)；Another road light enters circulation in the delay loop constituted that joined end to end by X-coupler as flashlight。Loop comprises the second frequency shifter (frequency shift amount Ω), fiber optic loop (time delay τ₀), erbium-doped fiber amplifier, fiber band-pass filter and Polarization Controller。In optical fiber loop, the every circulation primary of flashlight all shift frequency Ω time delay τ₀。After flashlight circulates m time in the loop, arrive second Y type bonder, and at its outfan with reference to photosynthetic beam interferometer, finally interference beat signal input detector and spectrum analyzer carry out acquisition of signal and analysis。When the delay inequality of the two difference path light beam is much larger than coherence time of laser, reference light and flashlight are equivalent to respectively from two independent light sources, and their beat signal of interfering just can correctly reflect the live width of measured laser device。

Step 2: utilize origin software that experimental data carries out Voigt matching, remove the measurement error that 1/f noise causes, revise measurement result。

The present invention avoids modulated interferer, it is as described below to put forward high-resolution operation principle:

As the laser beam of reference light after the first frequency shifter shift frequency Λ, electric field is represented by:

E₁(t)∝exp[jφ(t)]expj(Λt)(1)

Laser beam as flashlight enters optical fiber loop delay loop m rear (every circulation primary, signal light field time delay τ₀And shift frequency Ω), final signal light field is represented by

Wherein

c_n=γ^n/2exp[jn(n-1)Ωτ₀/ 2], n=0,1,2 ....

γ=α β

α is the coupling efficiency (0≤α≤1) of bonder 3, and β is the actual gain that loop is total。

Light field after bonder 2 place interferes superposition is

$\begin{array}{c}{E}_3\left(t\right)\∝E_1\left(t\right)+E_2\left(t\right)\\ \∝\exp \[j\mathrm{\φ}\left(t\right)\]\exp j\left(\mathrm{\Λ}t\right)+\underset{n=1}{\overset{\mathrm{\∞}}{\Σ}}{c}_n\exp \[j\mathrm{\φ}(t-{\mathrm{n\τ}}_0)\]\exp \left(jn\mathrm{\Ω}t\right),(n=0,1,2,\mathrm{...})\end{array}---\left(3\right)$

Being converted to photoelectric current through photodetector, photo-current intensity signal is

$\begin{array}{c}I\∝E_3\left(t\right)E_3^{*}\left(t\right)=2+\Σc_l{c}_l^{*}\\ +\underset{n=0}{\overset{\mathrm{\∞}}{\Σ}}\exp \{-j\[\mathrm{\φ}\left(t\right)-\mathrm{\φ}(t-{\mathrm{n\τ}}_0)\]\}\exp \[j(n\mathrm{\Ω}-\mathrm{\Λ})t\]+c.c\\ +\underset{m=1}{\overset{\mathrm{\∞}}{\Σ}}\left\{\underset{l=0}{\overset{\mathrm{\∞}}{\Σ}}{c}_l^{*}{c}_{l+m}\exp \right\{-j\[\mathrm{\φ}(t-{\mathrm{l\τ}}_0)-\mathrm{\φ}(t-(l+m\left){\mathrm{\τ}}_0\right)\]\}\exp \left(jm\mathrm{\Ω}\right)+c.c.\}\\ =term1+term2+term3\end{array}---\left(4\right)$

Wherein

$term1=2+\Σc_l{c}_l^{*}$ For DC terms,

For circulating beat frequency item between the flashlight of m time and reference light, center angular frequency is m Ω-Λ。

$term3=\underset{m=1}{\overset{\mathrm{\∞}}{\Σ}}\left\{\underset{l=0}{\overset{\mathrm{\∞}}{\Σ}}{c}_l^{*}{c}_{l+m}\exp \right\{-j\[\mathrm{\φ}(t-{\mathrm{l\τ}}_0)-\mathrm{\φ}(t-(l+m\left){\mathrm{\τ}}_0\right)\]\}\exp \left(jm\mathrm{\Ω}\right)+c.c.\}$

Differ the beat frequency item between the interfering beam of m time for circulation exponent number, mid frequency is m Ω。

(4) formula shows that photoelectric current comprises the beat signal of a series of frequency-distributed, and the center angular frequency of these signals is m Ω, m Ω-Λ (m rank beat signal), then the photoelectric current that m rank beat signal is formed is sized to

$\begin{array}{c}{I}_m\left(t\right)=\exp \[-j\mathrm{\φ}\left(t\right)-j\mathrm{\φ}(t-{\mathrm{m\τ}}_0)\exp \]\exp \[j(m\mathrm{\Ω}-\mathrm{\Λ})t\]\\ +\underset{l=0}{\overset{\mathrm{\∞}}{\Σ}}{c}_l{c}_{l+m}^{*}\exp \[j\left(m\mathrm{\Ω}\right)t\]\exp \{-j\[\mathrm{\φ}(t-{\mathrm{l\τ}}_0)-\mathrm{\φ}(t-(l+m\left){\mathrm{\τ}}_0\right)\]\}\end{array}---\left(5\right)$

Photoelectric current auto-correlation function is

$R_m\left(\mathrm{\&tau;}\right)=<I_m\left(t\right)I_m^{*}(t-\mathrm{\&tau;})>$

Being launched by above formula, after seeking its Fourier's series and going DC component, the power spectral density function of the photoelectric current that just can obtain m rank is

$\begin{array}{c}{S}_m\left(\mathrm{\&omega;}\right)=F\&lsqb;R_m\left(\mathrm{\&tau;}\right)\&rsqb;\\ =S_0(\mathrm{\&omega;},{\mathrm{m\&tau;}}_0,m\mathrm{\&Omega;}-\mathrm{\&Lambda;})+S_0(\mathrm{\&omega;},{\mathrm{m\&tau;}}_0,m\mathrm{\&Omega;})|\underset{l=0}{\overset{\mathrm{\&infin;}}{\&Sigma;}}{c}_l^m{c}_{l+m}\exp \left({\mathrm{jl\&omega;\&tau;}}_0\right){|}^2\\ =\frac{{\mathrm{\&gamma;}}^m}{{(1-\mathrm{\&alpha;})}^2}P\left(\mathrm{\&omega;}\right)S_0(\mathrm{\&omega;},{\mathrm{m\&tau;}}_0,m\mathrm{\&Omega;})+S_0(\mathrm{\&omega;},{\mathrm{m\&tau;}}_0,m\mathrm{\&Omega;}-\mathrm{\&Lambda;})\\ =S_0(\mathrm{\&omega;},{\mathrm{m\&tau;}}_0,m\mathrm{\&Omega;}-\mathrm{\&Lambda;})+S_s(\mathrm{\&omega;},{\mathrm{m\&tau;}}_0,m\mathrm{\&Omega;})\end{array}---\left(6\right)$

Wherein, S₀(ω,mτ₀, m Ω-Λ) and for circulating power spectral density (the mid frequency m Ω-Λ, time delay m τ of beat frequency between the flashlight of m time and reference light₀), S_s(ω,mτ₀, m Ω) differ between the interfering beam of m time for circulation exponent number or between m rank circulation light beam and optical fiber veiling glare, the power spectral density of beat frequency composes (center m Ω, time delay m τ₀), therefore, the beat signal (m Ω-Λ) on m rank separates with the interference (m Ω) of circulation light interfascicular, and its spectral pattern is no longer influenced by the modulation of periodic function, it is to avoid loop interference signal and the cross-couplings of beat signal, improves certainty of measurement。Work as τ_c< < m τ₀Time, reference light and flashlight are equivalent to respectively from two independent light sources, S₀(ω,mτ₀, m Ω-Λ) and for strict Lorentzian type, its spectrum width characterizes the live width (twice for measured laser live width) of laser。In an experiment, by-10dB the spectrum width of beat frequency curve divided byCalculate the live width obtaining testing laser device。

Below in conjunction with drawings and Examples, the present invention is described in more detail, but the invention is not restricted to this embodiment。

Embodiment:

As shown in Figure 1, the present embodiment forms by with lower component based on the system of circulation self-heterodyne method Laser Measurement device live width:

Laser instrument: Commercial fibers laser instrument, centre wavelength 1550nm, live width Δ ν is about about 1kHz (coming from product test report)；

Isolator: isolation 30dB, it is prevented that laser instrument is caused damage by light feedback；

Bonder 1:Y type, splitting ratio 10:90；

Bonder 2:Y type, splitting ratio 10:90；

Bonder 3:X type, splitting ratio 90:10；

Frequency shifter f1: acousto-optic modulator frequency shift value Λ=50MHz；

Frequency shifter f2: acousto-optic modulator frequency shift value Ω=40MHz；

Attenuator: regulate the luminous power of circulation light path, reduces fiber optic noise (such as parasitic noise) in optical fiber loop simultaneously；

Fiber optic loop: fiber lengths 20km, time delay 100us (τ₀=n L/c), decay 4dB, is wound on aluminum fiber reel by symmetric form；

Erbium-doped fiber amplifier (EDFA): adjustable gain, maximum gain 28dB, maximal input-7dBm, maximum noise 5dB, it is used for compensating laser power attenuation after fiber optic loop；

Fiber band-pass filter: centre wavelength 1550nm, bandwidth 50GHz, for rejective amplifier amplified spontaneous emission noise；

Polarization Controller: the polarization of control signal light field；

PD: photodetector, bandwidth is about 1.5GHz；

Audiofrequency spectrometer: in order to measure beat frequency power spectrum。

Position relationship and the signal flow direction of above-mentioned device are as described below: the light of measured laser device output is by after isolator, it is divided into two parts by first Y type bonder, a part (10%) is as reference light, second Y type bonder is entered: another part (90%) is as flashlight through frequency shifter 1 shift frequency, being divided into two bundles through X-coupler: a part of flashlight exports from c end, main flashlight then enters in the self-loopa light path being connected by X-coupler b, d two ends and formed。Loop comprises frequency shifter, fiber optic loop, erbium-doped fiber amplifier, fiber band-pass filter and Polarization Controller。In loop, the every circulation primary of flashlight all shift frequency 40MHz time delay τ₀。The flashlight circulating m time can be exported by the c end of bonder 3, and enter bonder 2 and carry out interfering beat frequency with reference light, obtains the optogalvanic spectra density of laser instrument finally by photodetector and audiofrequency spectrometer。Delay time (m τ when fiber optic loop₀) much larger than τ coherence time of laser_c=1/ π Δ ν, the optogalvanic spectra density detected just can reflect the live width of laser instrument。In order to reduce the outer noise of ring noises such as () sound, vibration, temperature, air-flows to test system dry around, whole device is put in the sound proof box being enclosed with heat-preservation cotton。

In traditional LC-RDSHI scheme, can system accurately be measured it is critical only that of ultra-narrow laser linewidth and set suitable systematic parameter。Fig. 2 show the impact on the 10th rank beat signal power spectral density of the different loop gains。During as reduced loop gain, interference effect can make beat signal the situations such as depression or spike (shown in Fig. 2 a, c) occur。Only parameter is suitable, and its power spectral pattern just can avoid interference with the modulation (shown in Fig. 2 b) of effect。The present invention is directly separating spectral interference item by optimizing contrived experiment scheme, saves selection and the debugging of parameter, directly obtains the high-order beat signal of high s/n ratio。As it is shown on figure 3, reduce the gain of loop, the spectral pattern of the 10th rank beat signal does not have significant change。

When self-heterodyne method measures kHz laser linewidth, long delay line can cause that Gaussian noise contribution becomes big so that the spectral line width directly read is more than true live width。In an experiment, utilizing origin software that experimental data carries out Voigt matching, removing the gauss component in spectral pattern, thus reducing measurement error。

Voigt function is represented by:

$I\left(\mathrm{\&nu;}\right)={\&Integral;}_{-\mathrm{\&infin;}}^{\mathrm{\&infin;}}G\left({\mathrm{\&nu;}}^{\&prime;}\right)L(\mathrm{\&nu;}-{\mathrm{\&nu;}}^{\&prime;}){\mathrm{d\&nu;}}^{\&prime;}$

$G\left(\mathrm{\&nu;}\right)=\frac{\sqrt{4ln2}}{\sqrt{\mathrm{\&pi;}}{\mathrm{\&Delta;\&nu;}}_{GF}}\exp \&lsqb;-{\left(\mathrm{\&nu;}\sqrt{4ln2}\right)}^2/{\left({\mathrm{\&Delta;\&nu;}}_{GF}\right)}^2\&rsqb;$

$L\left(\mathrm{\&nu;}\right)=\frac{{\mathrm{\&Delta;\&nu;}}_{LF}}{2\mathrm{\&pi;}}\frac{1}{{(\mathrm{\&nu;}-{\mathrm{\&nu;}}_0)}^2+{\left({\mathrm{\&Delta;\&nu;}}_{LF}\right)}^2/4}$

G is normalized Gaussian, Δ ν_GFFor its full width at half maximum, L is normalized Lorentzian type, Δ ν_GFFor its full width at half maximum。

When self-heterodyne method measures live width, assume that live width is to deriving from quantum noise (namely spontaneous radiation causes)。But at practical situation it is, the spectrum of most of super-narrow line width laser instrument is not strict Lorentz lorentz's type, but mix Lorentz lorentz's type of white noise and the Gaussian (such as pump noise, acoustic noise etc.) of 1/f noise class, this can cause the extra wide of self-heterodyne spectral pattern, presents a complicated Voigt spectral pattern。When laser linewidth is wider, it is necessary to fiber delay line comparatively short, Gaussian noise is little on measurement result impact, but when laser linewidth is more and more narrow, it is necessary to during very long delay line, Gaussian noise contribution becomes big, make the spectral line width directly read more than true live width, bigger error occurs。In this case, adopt Voigt spectral pattern Fitting Analysis, extracting the gauss component in spectral pattern, frequency spectrum being modified, thus reducing measurement error。Fig. 4 utilizes Voigt spectral pattern to be fitted 20 rank laser power spectrum densities analyzing, and obtaining laser Lorentz live width is 0.92kHz, is slightly less than the line width values that observation spectral pattern directly reads by experimentThe laser linewidth that experiment obtains is basically identical with laser beat frequency live width (deriving from product test report)。

Claims (5)

1. the accurate measurement laser linewidth device based on circulation self-heterodyne interferometric method, it is characterized in that, including narrow linewidth laser (1), described narrow linewidth laser (1) is connected with isolator (2), Y type bonder (3), the first frequency shifter (4), the second bonder (11), photodetector (12) and audiofrequency spectrometer (13) in turn by optical fiber；It is also associated with X-coupler (5) by optical fiber between described Y type bonder (3) and the 2nd Y type bonder (11)；Described X-coupler (5) also by optical fiber successively with attenuator (6), the second frequency shifter (7), erbium-doped fiber amplifier (8), fiber band-pass filter (9), protect inclined device (10) be connected formation closed circuit。

2. the accurate measurement laser linewidth device based on circulation self-heterodyne interferometric method according to claim 1, it is characterized in that, the a end of described X-coupler (5) is connected to the outfan of Y type bonder (3), and the c end of X-coupler (5) is connected to the input of the 2nd Y type bonder (11)。

3. the accurate measurement laser linewidth device based on circulation self-heterodyne interferometric method according to claim 1, it is characterized in that, the d end of described X-coupler (5) is connected to the input of attenuator (6), and the b end of X-coupler (5) is connected with the outfan protecting inclined device (10)。

4. the accurate measurement laser linewidth device based on circulation self-heterodyne interferometric method according to claim 1, the outfan of described second frequency shifter (7) is connected with the input of erbium-doped fiber amplifier (8)。

5. the method based on the accurate measurement laser linewidth of device described in claim 1-4 any one, it is characterised in that comprise the following steps:

Step 1: the light of measured laser device output is divided into two parts by Y type bonder (3), a part, as reference light, enters the 2nd Y type bonder (11) through the first frequency shifter (4)；Another part, as flashlight, enters in the self-loopa light path being connected by X-coupler (5) b, d two ends and formed；The time delayed signal light of n circulation is exported by the c end of X-coupler (5), and enter the 2nd Y type bonder (11) carry out from the reference light of different propagation paths interfere beat frequency, and by photodetector (12) and audiofrequency spectrometer (13) acquisition laser instrument optogalvanic spectra density；When the delay time of fiber optic loop is much larger than coherence time of laser, optogalvanic spectra density just can reflect the live width of laser instrument；

Step 2: utilize Voigt spectral pattern analysis, extracts the 1/f measurement error caused, revises measurement result。