Hello Ashrafi,

as for longitudinal modes measurement, you can utilize an optical spectral analyzer with high spectral resolution (such as COSA and BOSA with 5MHz Res.). In the situations that longitudinal intervals are ~MHz or ~100kHz, you may need to use a self-heterodyne interferometer to detect the longitudinal modes of laser under test.

On the other hand, for transverse modes measurement, a critical issue is how to distinguish these modes. I believe long-period fiber gratings can be used to achieve this goal, but I am sorry that I cannot send you a more favorable evidence to support this.

hi dear Hong,

Thanks for your response. This was very informative. I think we can use beam profiler to measure transverse modes of laser. am I right?

Dear Aydin

Many CMOS laser beam profilers, such as " Synology, Spiricom etc" can do transverse laser beam profiling nicely.

dawei

Dear Dawei,

thanks for your response.

Hello Ashrafi,

Of course a laser beam profiler can be utilized to measure transverse modes of laser.

In my last response, I was actually talking about how to distinguish the transverse modes of fiber laser. In this situation, these modes are strongly dependent on the special geometric structure of the fiber employed in laser configuration.

Thanks dear Hong

Using an optical spectrum analyzer and photo detector connected to an oscilloscope.

Dear Taha,

Thank you for your responce. can you describe more about mentioned method? for example : how the system work?

Bests,

Measuring longitudinal modes one should have a spectrometer with high spectral resolution, better special spectrometer designed for analysing laser radiation of your wavelength range. Transverse mode measurements include times diffraction limit (beam propagation factor) measurements, so called M2 (M squared) = 1/K. One could find beam profilers able to measure M2 from Spiricon (recommended above as Spiricom by mistake), Ophir or other companies specialised on laser beam measuring equipment. Beam profiling measurements are generally based on the standard ISO 11146-1:2005 and related.

Dear Mikhail,

I am thankful for your comment.

Bests,

Dear Aydin, longitudinal and transverse modes both have their characteristic frequency, however, they are typically too close and cannot be directly discriminated by average spectrometers, high precision wavelength-meter is a simple choice, but maybe too expensive, scanning Fabry-Perot spectrometer would be helpful if your source is cw or high repetition-rate pused, for single pulsed laser, a solid Fabry-Perot etalon with a CCD camera (or mobile phone) is OK. You can also find modes beat with fast a single detector and oscilloscope, just turn display button to FFT you'll find the modes beat peak, the precondition is that you know the resonator length and related longitudinal mode interval.

Dear Hongyan

Thanks for your informative answer.

Regarding longitudinal - many good answers.

The analysis of transverse modes is quite complicated. Good if only one mode (TEM with numbers like TEM00, TEM25) is present/generated - a beam profiler shows the intensity distribution. By min and max you could tell the mode numbers. Some are really beautiful. The problem starts when more and more transverse modes are present at the same time. For practical reasons M2 might be enough.

Dear Egidijus,

Thank you very much, you gave me another point of view. Could you explain how we can measure or do we have devices to measure such that more transverse modes?

Bests,

I have not seen anything like that. Most of my lasers were forced into a single mode operation. Others were with multi-longitudinal modes.

Dear Egidijus,

Thank you for your response.

Dear Aydin

The problem of characterizing the transverse modal content of beams with several modes is, as far as I know, not yet been completely solved. Several years ago Flávio Ferreira and I developed a means that is essentially an version of modal tomography using a spatial light modulator to generate a large set of transverse spatial modes to project onto the laser beam being characterized. More details can be found in " Holographic spatial coherence analysis of a laser ", Flavio Ferreira and Michael Belsley, Optics Letters, vol. 38, issue 21, p. 4350 (2013).

Good luck with your project!

Dear Michael,

Thanks a lot for your response. I downloaded your article and I am going to read it.

Bests,

Could anyone explain more about this question?

Dear Aydin ,

For longitudinal modes many has suggest suitable methods. For transverse modes, specially for lasers which may contain multiple transverse modes, there you can use a beam profiler (a CCD cam or Pyro CAM (high Power) and profile the beam at different positions and calculate divergence and further calculate M2 parameter which gives you an overall idea of beam quality. You can use commercial system for that also (look into Ophir photonics website). To exactly know the modal content of a laser output, I don't know such method which can directly do that. But, you can look into S2 measurement technique or modal decomposition technique. Also as suggested earlier, using spatial light modulators. You may find helpful information in the following articles -

1. Andrew Forbes, Angela Dudley, and Melanie McLaren, "Creation and detection of optical modes with spatial light modulators," Adv. Opt. Photon.8, 200-227 (2016)

2. J. W. Nicholson, A. D. Yablon, S. Ramachandran, and S. Ghalmi, "Spatially and spectrally resolved imaging of modal content in large-mode-area fibers," Opt. Express 16, 7233-7243 (2008)

3. J. W. Nicholson, J. M. Fini, A. M. DeSantolo, E. Monberg, F. DiMarcello, J. Fleming, C. Headley, D. J. DiGiovanni, S. Ghalmi, and S. Ramachandran, "A higher-order-mode Erbium-doped-fiber amplifier," Opt. Express 18, 17651-17657 (2010)

Dear Sourav,

Thank you for your answer. Could you please explain more about S2 measurement technique ?

Thanks,