LE-1000 Blue Diode pumped Pr:YLF Laser

Topics:

  • Blue Diode Laser 445 nm

  • Laser diode wavelength dependency

  • Pr:YLF Absorption spectrum

  • Pr:YLF Excitation spectrum

  • Lifetime of excited state

  • Hemispherical Cavity

  • Stability Criterion

  • Higher transverse modes

  • Extended Cavity

  • Littrow prism line tuning

  • Birefringent line tuning

  • Operating of green line

  • Active q-switch

  • Pockels cell

  • SHG 640 nm → 320 nm UV


LE-1000 Blue Diode pumped Pr:YLF LaserLE-1000 Blue Diode pumped Pr:YLF Laser

Due to the steadily increasing demand of the multimedia applications powerful RGB (red green blue) light sources came into the focus of industrial research. Along this road the Praseodymium laser has been reinvented again since this material has the potential to emit directly visible laser radiation on many interesting wavelength.
Energy level diagram of the Pr:YLF showing the absorption and emission
A. Energy level diagram of the Pr:YLF showing the absorption and emission
Whereas in the past this material has been of more scientific interest it is nowadays considered as a noteworthy candidate for RGB applications. The recent new developments of compact Pr:YLF laser have been enabled due to the presence of powerful blue emitting laser diodes. Such blue laser diodes actually have been developed for the powerful RGB data projectors. The aim of the experimental laser diode pumped Praseodymium YLF laser is to demonstrate these great potential as well as the exciting effect to study a four level laser system with visible radiation. The radiation of the blue emitting laser diode is collimated by the collimator which is a high precision aspheric lens with a short focal length and a high numerical aperture. The resulting beam is parallel in one axis showing a more or less rectangular to elliptical intensity cross section. A focusing lens is used to focus the blue pump laser radiation into the Praseodymium doped YLF (Pr:YLF)crystal which is coated with a broadband anti reflection coating on both sides, so called ARB coating. The wavelength range for lowest reflection covers the entire emission range of the Pr:YLF material including the pump radiation at 445 nm. The optical cavity is formed by a flat mirror on the pump side and a curved mirror at the other cavity side. The setup is designed in such a way that all components are accessible and can freely arranged on the optical rail. The measurement starts with the characterization of the blue laser diode which is connected to a digital controller allowing the settings of the injection current and temperature. In a next step the polarization dependent absorption is measured. The Pr:YLF crystal is set into a mount which is inserted into a to a 4 axis kinematic mount (PR) allowing the crystal to be rotated around its axis. As soon as the crystal is exposed to the blue radiation of the laser diode a bright white fluorescence appears. By means of an optional spectrum analyses the emission lines are identified. By using the modulation capability of the controller the lifetime of the excited state is measured and related to the Einstein coefficient for spontaneous emission. The most excited moment comes, when the laser cavity around the Pr:YLF crystal is setup. Due to the high gain, laser oscillation is obtained quite easy resulting in a strong red emission at 640 nm. To obtain laser oscillation also on other than the red line either a Littrow prism or birefringent tuner or mirrors with selective coating is used. Furthermore the generation of UV radiation with a wavelength of 320 nm as result of the intra cavity frequency doubling of the 640 nm line using a BBO crystal shows the great potential of such a laser system.
B. Pr:YLF Laser oscillating on the green line
B. Pr:YLF Laser oscillating on the green (523 nm) line using selective coated mirror M1 with a high reflectivity for 523 nm, but low reflectivity for the other. M2 has a radius of curvature of 100 mm or 150 mm with a broadband coating from 500 .. 750 nm.
C. Pr:YLF Laser oscillating on the red line selected by the BFT
C. Pr:YLF Laser oscillating on the red (640 nm) line using the intra-cavity birefringent tuner (BFT) with the same mirror M2 as in Figure B. However the flat mirror M1 is now also a broadband coated mirror with a high reflectivity in the range of 500 .. 750 nm.
Pr:YLF Laser with a spherical cavity
D. Pr:YLF Laser with a spherical cavity M1 is now a curved mirror with a radius of curvature (ROC) of 100 mm while M2 has a ROC of 150 mm. Both mirrors can also have the same ROC like 100 or 150 mm.
Pr:YLF Laser with a two flat mirrors M1 and M2
E. Pr:YLF Laser with a two flat mirrors M1 and M2. The intra-cavity lens (LI) has a focal length of 50 mm and is placed such that the laser beam's wave front becomes flat. Thus the mirror M2 is also a flat one and the cavity length can be expanded without restrictions.

A. Basic set-up with hemispherical cavity A. Basic set-up with hemispherical cavity

The Pr:YLF crystal is optically pumped with the blue (448 nm) radiation of a a laser diode (BLD). The emission is collimated by a short focal lens (CO) and subsequently focused with a lens (FL) having a focal length of 50 mm into the Pr:YLF crystal. The Pr:YLF crystal is located close to the beam waist of the optical cavity formed by the plane mirror M1 and the curved mirror M2. The Pr:YLF laser crystal is mounted into a rotary mount since the excitation efficiency depends on the polarization of the pump light. The filter (FI) is used to suppress not absorbed pump radiation, typically an RG495 color glass filter.


B. Set-up of a spherical cavity with two curved mirrors B. Set-up of a spherical cavity with two curved mirrors

By using two curved mirrors M1 and M2, a spherical cavity is realized. The Pr:YLF laser crystal is located apart from the in-coupling mirror M1. The mirror have a radius of curvature of 100 or 150 mm, so that also a combination of 100 and 150 mm can be used. In this case one needs to to take care about an optical instable range.
This configuration provides more space inside the cavity to accommodate more intra-cavity elements like second harmonic generation crystals (see Fig. C.)

C. Intra-cavity second harmonic generation of 640nm to 320 nm C. Intra-cavity second harmonic generation of 640nm to 320 nm

The strong radiation at 640 nm allows the efficient frequency doubling or second harmonic generation of UV radiation to 320 nm. Such UV radiation is of great importance in bio-photonics.
A LBO (Lithium triborate) crystal is used as frequency doubler. It is 8 mm long with a quadratic cross section of 3 mm. The crystal is cut for Type I phase matching and is applied intra-cavity.

D. Setup with two plane mirrors using an intra.cavity lens D. Setup with two plane mirrors using an intra-cavity lens

The cavity is extended by the lens (IL) resulting in an almost parallel beam required for the operation with the Littrow prism or of the plane mirrors. The Littrow prism (LP) is coated with a broadband coating having a high reflectivity >99.98 % in a range of 580..725 nm. 5 visible lines can be obtained 606, 639, 676, 697 and 720 nm.

E. laser line tuning with an intra-cavity birefringent tuner E. laser line tuning with an intra-cavity birefringent tuner (BFT)

The birefringent tuner (BFT) is inserted into the cavity between the intra-cavity lens (IL) and the flat mirror M2. 5 visible lines can be obtained 606, 640, 676, 697 and 720 nm.

F. Setup to measure the fluorescence of the optically pumped Pr:YLF rod F. Setup to measure the fluorescence of the optically pumped Pr:YLF rod

This simple set-up uses a photodiode and the filter FI to suppress not absorbed pump power. The controller can switch the diode laser (BLD) periodically on and off. This allows the measurement of the life time of the excited states.
LE-1000 Blue Diode pumped Pr:YLF Laser, consisting of:
Item Code Qty. Description
1 CA-0080 1 Optics cleaning set
2 CA-0450 3 BNC connection cable 1 m
3 DC-0040 1 Diode laser controller MK1
4 DC-0120 1 Si-PIN Photodetector, BPX61
5 MM-0020 3 Mounting plate C25 on carrier MG20
6 MM-0060 1 Filter plate holder on MG20
7 MM-0100 1 Target Cross in C25 Mount
8 MM-0110 1 Translucent screen on carrier MG20
9 MM-0440 1 Kinematic mount ø25.4 mm on MG20, left
10 MM-0462 1 Kinematic mirror mount M16, right
11 MP-0100 1 Optical Bench MG-65, 1000 mm
12 OC-0160 1 Collimator 445 nm in C25 mount
13 OC-0970 1 Filter GG495, 50 x 50 x 3 mm
14 OC-1130 1 Laser mirror M16, ROC flat, HT 445, HR 500-740 nm
15 OC-1134 1 Laser mirror M16, ROC 100, HT 445, HR 500-740 nm
16 OM-0052 1 Cylindrical Beam Expander 3x
17 OM-0622 1 Focusing optics, f=60 mm on carrier MG20
18 OM-0670 1 Pr:YLF crystal in 5 axis mount on MG20
19 OM-L446 1 Diode laser module 445 nm, 3 W in 5 axis holder
20 UM-LE10 1 Manual Pr:YLF Laser
Required Option (order separately)
21 CA-0200 1 Oscilloscope 100 MHz digital, two channel
Option (order separately)
22 CA-0270 1 Fiber coupled spectrometer 200 - 1200 nm, USB
23 LE-0820 1 Active Q-switch Extension
24 LE-1020 1 SHG 640 to 320 nm (UV) extension
25 LE-1030 1 Birefringent tuner extension
26 LE-1040 1 Littrow prism tuner extension
27 OC-S010 1 Set of mirror (ROC flat and 100 mm) for 520 nm operation
28 OC-S020 1 Set of mirror ( flat and ROC 100) for 604 nm operation
29 OM-0050 1 Cylindrical Beam Expander
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