Ultra-narrow linewidth laser spectroscopy measurement solution

Spectral linewidth measurements are to laser spectroscopy components. Different types of lasers, its value will have a width of several orders of magnitude difference: After continuous laser frequency stabilization especially its spectral linewidth can achieve 1Hz or less, and some lasers can cover a wide range of a few THz spectrum, such as femtosecond lasers.

Original value spectrum by Shawlow-Townes linewidth given expression, it shows essentially the width is a phase change caused by the spontaneous emission process caused. For the rare-earth doped lasers, such as erbium-doped fiber laser, Shawlow-Townes linewidth at mHz range value.

Generally, real narrow linewidth laser linewidth is difficult to measure: All measurements are subject to limited limited measurement time, and frequency caused by different noise sources during this period by the width of the laser light source Jitter, such as pump laser noise, acoustic noise, vibration noise. Narrow linewidth laser, for example, the measured linewidth can be seen as an integrated frequency jitter measurement system within the integration time due to technical noise source.

Linewidth measurement method

Self-heterodyne method:
Under normal circumstances, the use of self-heterodyne beat frequency measurement laser spectral linewidth. In this method, the signal via a two-way unbalanced Mach - Zehnder fiber interferometer, the way in which there is AOM frequency shifter, another way to delay fiber (Figure 1). For the measurement of narrow linewidth lasers, delay fiber length is usually 25km, corresponding to a time delay of about 120us. Two optical interference signal to produce a shape and width of the spectrum associated with the laser linewidth.

Self-heterodyne method
Self-heterodyne method

The measurement of spectral analysis, there are two cases: one is smaller than the coherence length of the laser interferometer arms or near poor, one is laser has a longer coherence length (the sub-coherent domain). Less than the coherence length of the laser interferometer arms difference under ideal conditions will produce a half-height, half-width equal to the width of the laser spectrum Lorentz spectrum (Figure 2).

Strictly speaking, the herein "ideally" refers only to the laser light having a white noise spectrum (corresponding to the coherence time of the exponential decay). Most very narrow spectral linewidth lasers contain a large amount of Gaussian noise (such as pump noise, vibration and noise, acoustic noise). This leads to a more complex Voigt line, it is a Gaussian and Lorentzian line linear convolution. For the rare-earth doped fiber laser, Lorentzian linewidth values ​​are generally small which is a linear function predominantly Gaussian line. This corresponds to the frequency of the noise spectrum in 1 / f function is presented in the form until the high frequency band (> MHz). Spectrum does not show the white noise floor, it's just by a trend / f function extends until the shot noise and ASE bands appear obvious. However, for the Lorentz line width, laser manufacturers are still often take a conservative measure, that measure since heterodyne linear 20dB below the peak point of the spectrum width, Gaussian influence here is not significant from the corresponding Lorentzian half-width also easily calculated, about 20dB 10% width.

Figure 3 illustrates the self-heterodyne laser linewidth measurement of C15: from heterodyne linewidth measured half-value width of approximately 32kHz, and 120kHz 20dB at half-width. Graphic display, the corresponding half width Gaussian curve 32 kHz, Lorentzian line function 20dB at half width 120kHz, corresponding Lorentzian line width of 12kHz; either Gaussian or Lorentzian line functions are not well matched and measured. Lorentzian values ​​intersect at only -20dB at Match difference clearly illustrates, using traditional methods and only this value as a Lorentzian laser linewidth measurement, because measured linewidth significantly narrower. As a comparison, the figure shows the fitted curve Voigt's.

For the coherence length is significantly greater than the difference between the laser interferometer arms, from the outside with a clear difference between the linear function linear function Lorentzian deviating. This is due to the coherent interference of light from the two paths of the interferometer. Figure 4 illustrates a case where the optical linewidth of 700Hz. Linear function of AOM by the frequency-dependent Dirac function and interferometer transfer function Δ composition is determined by the depth of the ripples of the laser linewidth. Measurement noise and limited system bandwidth and linear measurements will function theory and real ripples depth biased.

However, this type of linear function itself shows a line width of less than 1kHz, the best way to get the width of the measured data is a linear function fitting. Figure 5 illustrates the measurement of E15 laser linewidth (sub-coherent linewidth measurement), the corresponding value of the linear function corresponding to the line width is 200Hz.

Phase noise integration
Another method based on the measured linewidth frequency noise Score:
Frequency noise spectral density function

Here, S_Δθ (f) is the frequency noise spectral density function (press Hz2 / Hz basis). Although this method, at least in theory by the 1 / f noise is dominated by laser is effective, but only for the actual integration of the frequency range is known to be meaningful. And the use of optical fiber delay 25km from the outside compared to the beat frequency method, which should range from approximately 10kHz points until the upper frequency limit of the device. E15, frequency X15 (E15 frequency stabilized version) and C15 noise fiber laser shown in Figure 6:

Heterodyne beat linewidth

"Heterodyne beat linewidth" substantially covers the technical noise and frequency jitter caused by the width of the narrow-linewidth laser, the technology is the use of noise from the above described 25km delay fibers from the outer beat frequency kHz linewidth measurement value when due. Exact width measurement of these linewidth is very difficult, but is calculated based on the basic principles of the rare earth-doped fiber laser laser parameters showed that the value in the mHz range. Straightforward measurement method for obtaining these values ​​(as the name mentioned) is a laser with a stable measured narrow-linewidth laser light source or the like do beat. If you can use a sufficient resolution to capture the beat, the line width can be measured. Typical measurement obtained with the technical challenge of noise: Measurement sub-Hz line width desired value during the measurement can not drift out of the beat frequency range measurement window. For most lasers, this is a strictly limited, unless they can have very high stability. For example, the ORS1500 Menlo lasers, is an ultra-stable low thermal expansion locked fiber laser interferometer, which it produces a line width of less than 0.3Hz of the beat as possible (Figure 7, Figure 8). The disadvantage is that due to the size and complexity of the system, it is only for special applications only practical. 

For compact and stable laser X15 residual frequency drift suppression heterodyne measurement of having such a low value of the beat linewidth, but it was able to get only a few Hz linewidth values.

NOTE: coherence

Ideally, the coherence time of the laser linewidth is inversely proportional relationship between Δθ = 1/ (π ∙ τ_coh). This relationship only under strict conditions Lorentzian linewidth accurate. As mentioned earlier, for narrow linewidth laser, such as rare-earth doped fiber laser, the best measure of the line width can be seen as a more narrow linewidth integrated frequency jitter. Thus, if the measured line width is used, its coherence time (and the coherence length) is often much larger than the value obtained by the inverse relationship.

Laser linewidth summary

Laser linewidth summary
Laser linewidth summary

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