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    Mode Locked, mode locking

    Mode locking is where a fixed phase relationship between the modes of the resonant cavity arises. This relationship can then produce optical interference within the cavity, with the resulting laser emission occurring as a train of high energy pulses. Mode locking is used to produce pulsed outputs with picosecond or femtosecond durations.

    Modulation

    Analogue Modulation

    Analogue modulation enables the user to control the intensity of the output power. An external voltage signal, typically in the range of 0-5V, determines the laser output. The response of the laser power output to the input voltage is approximately linear.

    Digital Modulation (TTL Modulation)

    Digital modulation enables the user to modulate the output power of the laser diode digitally between zero power and maximum power via an external 5V TTL input.

    Available as a standard or optional feature on most of our lasers, this can also be used to switch the laser on or off remotely.

    The modulation frequency, Hz, refers to the speed at which the laser can be switched on and off.

    Pulse picker

    A pulse picker is an electrically controlled optical switch that blocks all but a specific single pulse in a pulse train.

    Q-Switching

    Q switching is a technique for obtaining energetic short pulses. The intracavity gain is modulated using a variable attenuator such as a saturable absorber or acousto-optic modulator. The technique generates nanosecond pulses of high energy and peak power.

    Monolithic

    Monolithic meaning "consisting of one piece" is typically applied to complex multiple optical elements that are provided as single elements.

    Monolithic optics offer a number of advantages over traditional air-spaced multiple optical element assemblies. They deliver greater stability and more resistance to drift of alignment, and provide greater resilience to shock and acceleration.

    Furthermore, they tend to be more compact, opening up opportunities for applications denied to larger multi-element systems.

    Monolithic Cavity

    A monolithic cavity is a laser cavity constructed from a single element, achieving greater alignment stability. This can reduce the effects of thermal drift, mechanical vibration and long term component reliability issues.

    MOPA Configuration

    MOPA stands for Master Oscillator Power Amplifier. A MOPA configuration combines a seed laser with an optical amplifier to boost output power whilst retaining many of the characteristics of the seed laser (such as the spatial mode, temporal mode, and coherence).

    Multimode Fibre Coupling (MM Fibre)

    Multimode fibre coupling allows numerous spatial modes and temporal modes of the laser beam to co-propagate through the fibre.

    It is particularly useful for short reach and low data rate optical communications, and low power beam delivery systems for applications such as sensing and optical pumping.

    When compared with singlemode fibre coupling, beam quality and coherence are compromised, but higher powers are achievable.

    Multimode fibres are available with core diameters of 50microns, 62.5 microns, 100 microns and greater.

    N

    Noise

    Optical noise manifests itself as variations mostly in the phase and amplitude of a laser. Generally unwanted, it can have several causes, some of which are easier to remove than others.

    All laser outputs exhibit some noise of varying degrees:

    Optical noise has many different sources: quantum noise from spontaneous emission in the gain medium; noisy drive electronics; interference from external EM sources; optical feedback; and environmental factors such as shock, vibration, and temperature fluctuations.

    Lasers operating on multiple resonator modes exhibit mode beating noise and mode partition noise, where the power distribution over the resonator modes fluctuates.

    Noise can also appear in the timing of the pulses (jitter), in the centre wavelength, the pulse duration, and frequency chirp.

    Phase & frequency noise cause laser linewidth broadening and limit the coherence.

    Amplitude noise has many serious effects in optical systems. Its effects can be far reaching: reducing the SNR in remote sensing and spectroscopy; degrading image quality in microscopy; and causing bit errors in datacoms.

    O

    OEM component

    An OEM component is a package format defined specifically for a particular product. It may be standard between several suppliers, but may not be recognised as a standard across the entire industry.

    Optical Amplifier

    An optical amplifier increases the power of an optical beam by use of an optical gain medium. The signal is amplified optically, and not converted into an electrical or electronic signal beforehand. This preserves many of the qualities of the laser.

    Optical bandwidth or laser linewidth

    The optical bandwidth (laser linewidth) is the range of optical wavelengths over which laser emission occurs simultaneously. Even a highly monochromatic laser source produces a range of wavelengths over which it emits simultaneously.

    Output Power

    Average Output Power

    This is the total power emitted by a pulsed beam, averaged over one period. For a pulsed laser delivering peak power per pulse of Ppeak with a pulse width of Pw and pulse repetition frequency of f, the average output power, Pave, is defined as:

    Pave = Ppeak x Pw x f

    Peak Output Power

    The maximum power of a train of identical pulses.

    This can be calculated from the average power and duty cycle (the fraction of the time period during which the laser is emitting):

    This can be expressed as:

    Ppeak (P0 on graphic) = Pavg/Duty Cycle

    Assuming a square pulse, Ppeak = Pavg / (Pulse rate * pulse duration (width))

    P

    Package Type

    Bar Array

    Several different versions of bar-array packages are available depending on the power and temporal characteristics of the laser emission. These mounts offer close packing and high density of the emitted laser radiation. They allow multiple arrays to be stacked together to produce many kW of integrated laser output for pumping or materials processing applications. Temperature control, and heat removal is important for these lasers to operate within specification and with long lifetimes. These mounts are designed to be used with a third party heatsink or temperature control (such as recirculating water, Peltier effect TECs or forced air, or passive conduction).

    Bench top

    A bench top laser comprises a laser head and a freestanding power supply unit. The power supply unit (PSU) incorporates control electronics, matched to the laser head. A custom cable is provided to make the connection between the laser head and PSU. Extremely user friendly, this laser format is ready to use once plugged into a mains supply.

    All of our laser PSUs are suitable for use with standard 240V AC (single-phase) mains electricity. They are supplied with UK mains plugs, and are CE marked. The laser heads have through-holes for mounting to standard optical tables or rail systems. Additionally, we provide the following options where they are not standard:

    • Manual optical shutter
    • Emission indicator light
    • Heat sink
    • Forced air cooling
    • Collimation or focussing optics
    • Line generation optics
    • Optical fibre coupling

    The PSU is designed to be freestanding on a lab bench. Higher power laser systems use larger PSUs and may be built into a 19-inch rack. This could require an optional rack mounting kit. Please refer to the data sheet for details.

    In the SK Group range, many of the following PSU features are standard, depending on the laser model:

    • Key activated on/off switch
    • On/off indication light
    • Safety interlock
    • Manual power amplitude adjustment
    • Output power level readout
    • Built in modulation control
    • RS232/IEEE/USB PC interface
    • External modulation input
    • 240VAC single phase power
    • 19 inch rack mounting kit
    • EU mains power plug

    The relevant datasheet should list the laser head and PSU features available. If you have a specific requirement for any of the features listed above and can't find the information you need, please contact us and we will be happy to help you.

    Butterfly

    The butterfly package is an industry standard hermetically sealed laser diode package. The BFY package is comprised of laser welded gold plated Kovar. This has very similar expansion characteristics to the borosilicate glass used in the frit seals and optics to deliver the ultimate in robustness and reliability. The package is also provided with a fibre pigtail (either singlemode or polarisation preserving with an FC/PC or APC connector). Horizontal pins arranged symmetrically on both sides are provided for connection to a suitable laser diode & TEC controller. Several different pin-out configurations are provided as standard. Most options include: a Peltier effect TEC and thermistor for temperature measurement, control, and stabilisation; and a monitor photodiode for output power measurement, power control and mode hop free performance.

    C-mount

    The C-mount is a format of open laser diode chip mount. It is designed to remove heat from the laser diode and be cooled actively by adequate heat-sinking to a 3rd party supplier cooler. The C-mount is available in a number of different formats.

    Compact OEM

    Our Compact OEM laser format combines the laser resonator and PSU in the smallest of all of our laser packages. Size is kept to a minimum as no power source is built in. Connection to a suitable DC voltage source is made via flying leads.

    Due to the small size, fewer features are available as options with this format. However, it is possible to provide the following:

    • Adjustable optics
    • Line generation optics
    • Alternative electrical connection options

    These laser heads are CE marked in accordance with CEI 60825.

    OEM component

    An OEM component is a package format defined specifically for a particular product. It may be standard between several suppliers, but may not be recognised as a standard across the entire industry.

    Rack Mount (19")

    The 19" rack mounting system is a space saving method of packing instrumentation into a highly compact tower. Each of the racks is 19" wide and is stacked vertically in industry standard unit sizes of n x 1U height (where 1U = 1.75"(44.5mm)).

    Self-Contained Module

    Lasers provided in a self-contained module have the power supply, control electronics and laser resonator combined together in a single compact package. A user replaceable power source (such as an AA or AAA battery) is also incorporated. No additional external power is necessary.

    Along with an on/off switch, the following features are available:

    • Adjustable focussing optics
    • Line generation optics

    These lasers are CE marked in accordance with CEI 60825.

    Substrate

    In this case, the laser diode is mounted directly to a ceramic or metallic sub-mount without any other packaging. Connection is made by soldering flying leads to, or making wire bonds to the substrate.

    TO

    Hermetically sealed laser diode chip package. 'TO' stands for Transistor Outline. The number refers to the case style. For example TO-39 is a metal 'can' package for semiconductor devices, sealed to protect the device from moisture and contaminants.

    Peak Output Power

    The maximum power of a train of identical pulses.

    This can be calculated from the average power and duty cycle (the fraction of the time period during which the laser is emitting):

    This can be expressed as:

    Ppeak (P0 on graphic) = Pavg/Duty Cycle

    Assuming a square pulse, Ppeak = Pavg / (Pulse rate * pulse duration (width))

    Phase

    The phase of a wave indicates the current position of the wave relative to a reference position. Phase is normally measured in degrees, where 360 degrees represents a single cycle of one wavelength.

    Two waves propagating with zero phase difference (0 degrees) are said to be in phase with each other and produce constructive interference.

    Similarly, waves propagating out of phase produce destructive interference, with maximum at 180 degrees phase difference.

    Polarisation Of Laser Emission

    Laser polarisation refers to the direction of oscillation of the electric field. Many lasers produce polarised outputs, the gain or resonator losses may be polarisation dependent.

    Those that do not give a polarized output, such as many fibre lasers, do not necessarily have a truly unpolarised output. Rather than the two components having equal powers at any time, it may simply be an unstable polarisation state that is constantly changing at high frequency.

    Polarisation Preserving Fibre coupling / Polarisation Maintaining Fibre Coupling (PM Fibre)

    Polarisation preserving fibre coupling is a form of singlemode fibre coupling. It uses a special type of fibre where the structure of the fibre core is designed to ensure that a single linear polarisation state is propagated along the length of the fibre.

    Polarisation Ratio

    The measure of polarisation, 'polarisation extinction ratio' quantifies the degree of linear polarisation. It is the ratio of optical powers in the maximum and minimum axes of the elliptical representation of the beam profile. This is effectively the same as the ratio of the transmission of the unwanted component to the wanted component.

    Pulse compression / Pulse Expansion

    Pulse compression reduces the pulse duration of an optical pulse. Various methods are used depending on the type of source. Most solutions use some form of grating or non-linear optical effect.

    Pulse Duration (Pulse Width)

    This is the duration of a single pulse of a pulsed laser, and is the time during which the pulse output power is above one half of its maximum value (typically). This is known as full duration at half maximum (FDHM).

    Pulse Energy

    The optical energy contained within a single laser pulse. It is often calculated by dividing the average output power (Pave), by the pulse repetition frequency (f, Hz)

    Epulse = Pave/f

    It is also possible to calculate from the peak power of a laser, provided that the peak power is expressed per pulse and has highly uniform over the pulse duration.

    Epulse = Ppeak x pulse duration

    Pulse picker

    A pulse picker is an electrically controlled optical switch that blocks all but a specific single pulse in a pulse train.

    Pulse Repetition Frequency

    Also known as the pulse repetition rate, this is the number of pulses emitted per period. When the period is 1 second, the pulse repetition frequency is given in Hertz.

    Pulsed (laser output)

    A pulsed laser emits light in the form of a single pulse or a train of optical pulses. A pulsed laser is defined by temporal characteristics such as the duration of each pulse (pulse width/pulse duration) and the rate at which pulses are repeated (pulse repetition frequency). Pulsed lasers may emit pulses that display do not exhibit uniform pulse rates, durations or energy. This is often described as jitter.

    Pumping

    End Pumping

    Pump light is injected into the gain medium in the same axis as the propagation axis of the laser emission.

    Side Pumping

    Pump light is injected into the gain medium at an orthogonal axis to the propagation axis of the laser emission.

    Q

    Q-Switching

    Q switching is a technique for obtaining energetic short pulses. The intracavity gain is modulated using a variable attenuator such as a saturable absorber or acousto-optic modulator. The technique generates nanosecond pulses of high energy and peak power.

    QCW operation (Quasi-Continuous Wave)

    A QCW laser emits a stream of pulses at a high frequency. The peak and average powers of the laser are of similar magnitudes and the laser behaviour approximates to that of a CW laser.

    Quantum Cascade Laser / External Cavity Quantum Cascade Laser

    A Quantum Cascade Laser (QCL) is a semiconductor laser that emits highly coherent radiation in the mid- to long-wave infrared region of the spectrum. QCLs are not diode lasers, but rather unipolar semiconductor devices consisting of hundreds of epitaxial grown layers forming a large number of quantum wells in the conduction band of the device. These are engineered to enable a cascade of photons emitted for each injected electron. QCLs generate light in the 4 µm to 25 µm region of the electromagnetic spectrum.

    An External Cavity Quantum Cascade Laser (ECqcL™) is a semiconductor laser source. It integrates quantum cascade gain media into an external cavity having wavelength dependent feedback. ECqcLs™ are available either as precision fixed-wavelength sources, or as broadly tunable lasers. A tunable ECqcL™ can tune across the entire gain profile of the QC chip, allowing for tunability of 10% to 25% of the center wavelength

    Quantum Efficiency

    The quantum efficiency of a photo-detector or camera is defined as the number of photon collisions that are required to produce and electron-hole pair, and relates to the overall sensitivity of the detection system.

    R

    Rack Mount (19")

    The 19" rack mounting system is a space saving method of packing instrumentation into a highly compact tower. Each of the racks is 19" wide and is stacked vertically in industry standard unit sizes of n x 1U height (where 1U = 1.75"(44.5mm)).

    Rayleigh Length

    The Rayleigh Length is the distance from a beam waist at which the mode radius has increased by a factor of √2.

    Resonator Modes: Transverse (Spatial) Mode and Longitudinal Mode

    Modes of an optical or microwave cavity (resonator).

    Due to the effect of interference on light confined within a cavity, only certain spatial patterns and frequencies are sustained. Radiation patterns that reproduce themselves on each round trip through the resonator are the most stable and form the modes.

    Longitudinal modes differ in frequency, transverse modes in both frequency and intensity pattern.

    The fundamental transverse mode of a resonator, TEM00, is a Gaussian beam.

    S

    Seeding (Injection)

    Injection seeding is a method used to force narrowband operation of a laser. The technique involves coupling light from a seed laser into the resonator of a slave laser. If the frequency of the seed light is close enough to the resonant frequency of one of the modes of the slave laser, that particular mode will oscillate in preference over the others and will extract much higher power than the competing modes. This mode will dominates the output, reduce the emission bandwidth, and give a smoother temporal pulse profile.

    Self-Contained Module

    Lasers provided in a self-contained module have the power supply, control electronics and laser resonator combined together in a single compact package. A user replaceable power source (such as an AA or AAA battery) is also incorporated. No additional external power is necessary.

    Along with an on/off switch, the following features are available:

    • Adjustable focussing optics
    • Line generation optics

    These lasers are CE marked in accordance with CEI 60825.

    Side Pumping

    Pump light is injected into the gain medium at an orthogonal axis to the propagation axis of the laser emission.

    Singlemode Fibre Coupling (SM fibre)

    Singlemode fibre coupling preserves the spatial mode and temporal mode of the laser beam by allowing only one (single) mode to propagate through the fibre. However, it does not preserve the polarisation state of the fibre, which is randomised.

    It can be used to convert multi-spatial mode beams into single-spatial mode beams at the expense of increased loss.

    It is particularly useful for optical communications, distributed sensors, and applications where excellent beam quality or coherence is required.

    Singlemode fibres tend to have the smallest dimensions, with core diameters typically less than 10 microns.

    Slope Efficiency (Differential Efficiency)

    Slope efficiency of a laser is defined as the ratio of laser output power versus the input pump power. As this value is generally linear for most lasers operating in the lasing region, it is sufficiently expressed as a single ratio. However, at conditions below laser threshold and close to peak output power, the variation of output power with input power becomes non-linear and this approximation is no longer accurate.

    Solid State Laser

    Solid state lasers are based around a solid gain media such as ion-doped crystals or glasses, (e.g. Nd:YAG, Nd:YLF, Ti:sapphire, Nd:YVO4 etc...).

    These lasers have the ability to deliver highly coherent radiation in the UV, visible or Infrared ranges of the spectrum, in pulsed or CW forms and with high optical output power and quality. They also offer the ability to operate in highly singlemode states for applications requiring long and stable coherence.

    High powers can also be achieved, making them suitable for some materials processing applications.

    Substrate

    In this case, the laser diode is mounted directly to a ceramic or metallic sub-mount without any other packaging. Connection is made by soldering flying leads to, or making wire bonds to the substrate.

    Superluminescent Light Emitting Diode (SLED)

    SLEDs are semiconductor devices that incorporate high-power gain sections to give amplified spontaneous emission. Similar in construction to laser diodes, but without optical feedback to cause laser action, they combine the high power and brightness of laser diodes with the low coherence of conventional light-emitting diodes.

    T

    Tapered Amplifier

    A tapered amplifier is a semiconductor optical device designed to amplify the signal of a low quality semiconductor source and to improve the resulting beam quality. The angle of the taper, and the length of the unamplified region are optimised to deliver high quality beams with low ASE noise.

    Temporal mode

    Continuous Wave, CW (laser output)

    CW lasers emit laser radiation constantly with time. The output power may be adjusted by the source or by the use of attenuators in the beam path, but emission continues until the laser is switched off or reaches the end of its lifetime.

    Pulsed (laser output)

    A pulsed laser emits light in the form of a single pulse or a train of optical pulses. A pulsed laser is defined by temporal characteristics such as the duration of each pulse (pulse width/pulse duration) and the rate at which pulses are repeated (pulse repetition frequency). Pulsed lasers may emit pulses that display do not exhibit uniform pulse rates, durations or energy. This is often described as jitter.

    QCW operation (Quasi-Continuous Wave)

    A QCW laser emits a stream of pulses at a high frequency. The peak and average powers of the laser are of similar magnitudes and the