LE-0300 HeNe-Laser advanced


  • HeNe Energy Level Diagram

  • ABCD Law & Resonator

  • Optical Gain

  • Optical Stability Criteria

  • Cavity Alignment

  • Output Power vs Discharge Current

  • Gaussian Beams

  • Line & Mode Selection

  • Crystal Optics

  • Birefringent Filter

  • Spectrum Analyser

  • Free Spectral Range

  • Lamb Dip

  • Intra-Cavity Iodine Cell

LE-0300 HeNe-Laser, advancedLE-0300 HeNe-Laser, advanced

The humble Helium Neon (HeNe) laser still has many applications, due to its superior beam quality and coherence. In all physics text books this laser represents the class of the gas laser and was the first gas laser invented by Ali Javan in 1960 right after Theodore Maiman demonstrated the first operation of the ruby laser. Since the HeNe laser was continuously operating and easy to build in a laboratory it served as specimen for a lot of scientific work and proof for theoretical predictions. It starts with the theory of optical resonator, Doppler broadened laser active material in a cavity, spectral hole burning (Lamb dip), single mode operation, coherence and intra-cavity absorption (inverse Lamb dip) just to name a few. For technical applications the HeNe laser is still in use due to its outstanding beam quality and coherence as secondary meter standard and is present in each air plane or ships as laser gyroscope for navigation. This experiment is designed as an open frame setup in such a way that all components can be arranged freely on a stable optical rail. A Helium Neon tube with Brewster windows on both ends is used to perform a variety of fundamental experiments. Verification of mode selection properties, the optical stability range and the ABCD matrix formalism of the cavity used are discussed. A birefringent filter as well as a Littrow prism is used for the wavelength selection and the effect of an etalon used inside the cavity are investigated. A photo detector for measuring the relative output power and an alignment laser are supplied with a 1 meter long optical rail, along with all necessary mounts and adjusters. For the visualization of the mode structure a „Fabry Perot“ extension is available or an electronic spectrum analyser is used to measure the modes beat frequency. The optical resonator is formed by two precision adjustment holders for common 1/2 inch exchangeable mirrors having different radii of curvature. For ease of adjustment, at the beginning a "green" pilot laser is attached as an alignment aid. The laser tube is mounted into XY-adjustments to align the tube with respect to the pilot laser.

LE-0300 HeNe-Laser, advanced LE-0300 HeNe-Laser, advanced

A glass tube (LT) is terminated on both sides with Brewster windows and contains an optimized mixture of Helium and Neon gas. The adjustable mirror M1 and M2 are forming the optical cavity or resonator. The main difficulty to operate the HeNe laser experiment is the first alignment of the optical cavity formed by the mirror M1 and M2. To line up the laser tube and the mirror with respect to the mechanical axis of the optical bench, defined by the iris diaphragm (IR), a green emitting laser pointer (PL) is provided. In addition we designed the coating of the laser mirror in such a way that a certain reflectivity laser pointer emission is obtained. In case both Fabry Perot Interference fringes mirrors are aligned parallel to each other they form a Fabry Perot and the transmitted beam shows the typical interference fringes indicating the perfect alignment.
To select different lines of a laser a Littrow prism is used (LP). Within this experiment we are using such a module to tune the Helium Neon laser. The Littrow prism is made from fused silica which is the required substrate for IBS coating. The spectral range of the coating covers 580..720 nm with a reflectivity >99.98 %. The prism is mounted into a precise adjustment holder where it can be smoothly tilted in vertical or horizontal direction.
A double refractive or birefringent plate is mounted in a dual rotational stage (BF). For the intra-cavity operation the birefringent plate needs to be aligned in such a way that the laser beam hits the plate under the Brewster angle to minimize the reflection losses. This is done turning the rotary plate . In addition the birefringent plate can be rotated around its optical axis by tilting the lever. Rotating the plate its optical retardation δ is changed. If the retardation of two passes is a multiple integer of the wavelength λ, this wavelength undergoes no losses at the Brewster window and will oscillate.
Due to the in-homogeneously broadened gain profile of the Neon laser transition, a multitude of laser modes are oscillating simultaneously. These modes have a frequency difference to each other which depends on the length of the Helium Neon laser cavity given by the distance between mirror M1 and M2. By means of the provided fiber coupled photodetector (PD), photodetector box and an electronic spectrum analyser this beat frequency can be measured as a function of the cavity length. The value of the beat frequency for a 750 mm long cavity is150 MHz for instance. To obtain single mode operation, the etalon (ET) is used. Its efficacy is checked either with the electronical or Fabry-Perot spectrum analyzer.
The photodetector (PD) along with the photodetector box is also used to measure optical power which is displayed on a digital voltmeter.
The screen (SC) is used to display the transverse modes.

LE-0300 HeNe-Laser, advanced, consisting of:
Item Code Qty. Description
1 CA-0080 1 Optics cleaning set
2 CA-0220 1 Multimeter 3 1/2 digits
3 DC-0060 1 High voltage supply 4.0 - 7 mA adjustable
4 DC-0300 1 Biased Photodetector 1.5 GHz
5 LQ-0030 1 Green (532) pilot laser with USB power supply
6 MM-0020 1 Mounting plate C25 on carrier MG20
7 MM-0230 1 Photodetector mount on rotary arm on MG20
8 MM-0420 1 Four axes kinematic mount on carrier MG20
9 MM-0460 1 Kinematic mirror mount M16, left
10 MM-0462 1 Kinematic mirror mount M16, right
11 MP-0100 1 Optical Bench MG-65, 1000 mm
12 OC-0400 1 Adjustable iris mounted in C25
13 OC-1000 1 Laser mirror M16, flat, T 3% @ 632 nm
14 OC-1005 1 Laser mirror M16, flat, HR @ 632 nm
15 OC-1020 1 Laser mirror M16, ROC 700 mm, HR @ 632 nm
16 OC-1030 1 Laser mirror M16, ROC 1000 mm, HR @ 632 nm
17 OC-2504 1 Plastic optical fibre FSMA, length 1 m
18 OM-0560 1 HeNe laser tube with XY and wobble alignment
19 OM-0570 1 Littrow Prism Tuner
20 OM-0580 1 Birefringent Tuner
21 OM-0590 1 Single Mode Etalon with kinematic mount
22 OM-0596 1 Transverse Mode Enhancer
23 UM-LE03 1 Manual HeNe Laser
Option (order separately)
24 CA-0060 1 Infrared display card 0.8 -1.4 µm
25 CA-0200 1 Oscilloscope 100 MHz digital, two channel
26 CA-0210 1 Spectrum Analyzer 100 kHz - 500 MHz
27 CA-0270 1 Fibre coupled spectrometer 200 - 1200 nm, USB
28 CA-0510 1 Laser safety goggles for LE-0300 HeNe Laser
29 LE-0350 1 HeNe Fabry Perot Mode Analyser
30 OC-1040 2 Laser mirror M16, ROC 700 mm, HR @ 1180 nm
31 UM-LM03 1 Manual Fabry Perot Resonator
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