A Gaussian beam laser often has an internal aperture to restrict higher modes leaving only the lowest order TEM00. If this is the case for your Ti:saph laser, the donut mode can often be obtained by simply replacing the aperture with one having a larger hole. You should get considerably greater laser output power but the beam divergence will also be significantly larger. Greater power can give you more SHG but the greater divergence will produce a larger focal spot which could reduce SHG. It depends on which dominates.
I don't have much experience with axicon lenses but I am skeptical about how effective it would be. The axicon may indeed produce a donut shaped beam in the near field but in the far field and therefore the focus of your lens, you may lose your donut and you may no longer be close to the diffraction limit making the SHG much worse.
Lauren M. Peterson, PhD Lead Physicist - Photonics
Michigan Research & Development Center
General Dynamics - Advanced Information Systems; Integrated Space Systems
1200 Joe Hall Dr., P.O. Box 990 Ypsilanti, MI 48197
734-480-5303 -480-5252 FAX ISS-SIS-ATD-EPA
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Demetrius Ypsilanti fought in the Greek war for independence against the Ottoman Empire and led the culminating Battle of Petra in 1829.
-----Original Message-----
From: Confocal Microscopy List [mailto:[log in to unmask]] On Behalf Of Adam Larson
Sent: Thursday, August 20, 2009 4:05 PM
To: [log in to unmask]
Subject: Re: laser profiles
The annular mode I think being referring to is a TEM01 (Laguerre-Gaussian) mode. This mode solves the propagation equation when using cylindrical coordinates. It is often referred to as a doughnut or bagel mode. These modes have a profile similar to a Bessel beam. In fact, a Bessel beam can be created by passing this mode through an axicon. Bessel beams are known to be non-diffracting (at least a perfect Bessel beam). This beam profile can be used to greatly enhance the lateral resolution of an imaging system. I presume this is the reason you are interested in the mode. The tighter focusing will allow higher SHG signal by improving the phase matching. This mode profile should not have any negative effects on SHG or two-photon fluorescence in biological samples. It may actually help generate higher signals and improve lateral resolution. Of course, the tighter focusing may require you to reduce the on sample power to reduce nonlinear photodamage. As far as I know the drawback to these beams is a reduction in the axial resolution. If I remember correctly, this results from the axial component of the polarization vector in the mode as it is focused under high NA. To make designing a laser a little bit easier since I believe that the Laguerre-Gaussian modes are a little bit more difficult to obtain than the usual Hermite-Gaussian mode profiles (misalignment, dirt and strain on optical surfaces tends to push lasers into propagation modes solved by rectangular coordinates), can you use a Gaussian TEM00 mode passed through an axicon creating a circular beam profile?.
Regards,
Adam M. Larson, Ph.D.
Advanced Imaging Group
435 Route 206
Newton, NJ 07860
Tel: (973)300-4497
THORLABS Inc.
-----Original Message-----
From: Confocal Microscopy List [mailto:[log in to unmask]] On Behalf Of Craig Brideau
Sent: Thursday, August 20, 2009 3:29 PM
To: [log in to unmask]
Subject: Re: laser profiles
Isn't TEM10 a double-lobed thing? I think shearing will generate that mode.
Craig
On Thu, Aug 20, 2009 at 1:01 PM, Jerry (Gerald)
Sedgewick<[log in to unmask]> wrote:
> I have been requested to provide a non-gaussian profile for the laser output
> on our Ti:Sapphire laser that is used on our custom built multiphoton
> devices. This profile--apparently more of an annulus--works best for
> obtaining SHG on nano-sized features introduced onto metal surfaces. The
> profile is referred to as TEM10 (versus TEM00, which is a gaussian profile).
> Since I am hoping to add an additional laser to the system, my desire is to
> provide a laser system in which either the TEM00 or TEM10 profile can be
> chosen, if this is even a possibility.
> Barring that, can the TEM10 profile be used on standard biological samples
> for both SHG and standard excitation of fluorophores, or will the profile
> affect the results (e.g., will resolution or some other characteristic
> change)?
>
> If anyone has had experience with the TEM10 profile, I'd appreciate
> feedback!
>
> Jerry Sedgewick
> Program Director
> Biomedical Image Processing Lab
> University of Minnesota
>
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