Ti:sapphire or ytterbium-doped? How to choose the femtosecond laser that better suits your needs
In the field of exploring femtosecond laser technology, a comparative study of titanium sapphire (Ti:Sapphire) and ytterbium-doped (Yb) lasers reveals the unique advantages and potential applications of both systems. From the birth of the earliest mode-locked lasers to the development of modern all-solid-state self-mode-locked femtosecond lasers, technological advancements have pushed the boundaries of physics research, to the revolutionary role of titanium sapphire lasers in the study of ultrafast phenomena physics, to the ytterbium-doped crystal lasers due to their high efficiency and low By comparing the technical principles, performance parameters, applicability and their positions in the market of these two laser systems, we aim to provide guidance to scientific researchers and engineers to help them choose the most appropriate femtosecond laser technology path according to their specific needs.
The generation of ultrashort pulses is based on the phase locking of multiple longitudinal modes in the laser cavity. When these modes are locked together, structural interference will occur, resulting in the generation of short pulses. These lasers are so-called mode-locked laser oscillators. Since the first generation of mode-locked lasers in 1964 [1,2], many significant changes have occurred. The first major evolution occurred in the 1970s and 1980s with the use of femtosecond dye lasers [3,4]: this was the second generation of mode-locked lasers. However, perhaps the most important progress relates to the breakthrough in the early 1990s, when a new solid laser medium for mode-locked lasers emerged: titanium-doped sapphire [5,6]. This event marked the beginning of the third generation of femtosecond lasers [7]: all-solid-state self-mode-locked femtosecond lasers. This amplifier system, which combines mode-locked femtosecond laser with chirped pulse amplification technology (CPA) [8,9], has greatly improved in parameter performance, efficiency and functionality, and has completely changed the physics research of ultrafast phenomena. [10]
The working principle of a laser amplifier begins with the pumping process, which usually requires an intense continuous wave laser or another pulsed laser as the pump source. This pump source is used to excite electrons in the gain medium (Ti:Sapphire/Yb crystal, etc.) to a higher energy level. When the electrons reach an excited state, they release energy as a low-energy femtosecond pulse passes through the crystal, amplifying the pulse. They have been able to generate ultrashort ultrahigh power (up to 1015W) pulses, which corresponds to the beginning of ultrahigh intensity physics and the massive expansion of femtosecond laser chain applications.
When choosing a gain medium for femtosecond laser amplifiers, there is often a trade-off between titanium sapphire (Ti:Sa) and ytterbium-doped (Yb) materials. Breakthroughs brought about by various new crystals that generate ultrashort pulses have revolutionized ultrafast laser technology and its applications. The first revolution occurred in the 1990s with the development of the first compact, efficient and reliable titanium sapphire all-solid-state mode-locked lasers; the second revolution - due in particular to new Yb-doped crystals - involved the development of very efficient directly diode-pumped femtosecond lasers. These new generation mode-locked lasers have attracted great attention as their applications have rapidly expanded. However, there may still be many questions about how to select these two lasers. In the following, we will introduce the two mainstream types of femtosecond laser amplifiers (Ti:Sa/Yb) in the market from the perspectives of technical principles, market products, etc.
1. Titanium sapphire femtosecond (Ti: Sapphire)
Titanium sapphire femtosecond laser amplifiers, often called titanium sapphire femtosecond lasers, are used to amplify the energy of very short light pulses. This laser uses titanium-doped sapphire (Ti:Sapphire) crystal as the gain medium, and titanium-doped sapphire (Ti:Sapphire) crystal has excellent laser performance. We can easily handle titanium sapphire as it is highly stable in humidity, properly obtains high gain of amplification due to sufficient emission cross section and can also be used in high energy amplifiers as it can carry large thermal load due to high thermal conductivity. However, the most obvious feature of titanium sapphire is that it has the widest gain bandwidth of any solid-state laser material, providing a spectrum broad enough to generate sub-10 femtosecond light pulses.
Titanium: sapphire crystal
The Ti:sapphire amplifier system mainly consists of several parts: the pump laser provides the necessary energy to excite the Ti:sapphire crystal to generate the initial femtosecond pulse and serves as an oscillator for the amplification process; the pulse stretcher extends the pulse width before amplification to prevent crystal damage; the amplifier itself contains the Ti:sapphire crystal and is responsible for amplifying the pulse; finally, the pulse compressor compresses the pulse back to its original or shorter duration after amplification.
Ti sapphire femtosecond pulse generation and chirped pulse regeneration amplification experimental structure diagram
FR is Faraday rotator PC is PockelscelI box TFP is thin film polarizer [11]
The technical features of titanium sapphire femtosecond laser amplifiers include wide wavelength tuning range, ultra-short pulse width, and high peak power. Its wavelength can usually be tuned in the range of 700 to 1000 nanometers. In order to obtain the required pulse width and energy, the pulse can be accurately compressed and further amplified in the subsequent amplification process. The titanium sapphire femtosecond laser amplifier can provide ultra-high peak power. Although its average power is not necessarily very high, its ultra-short pulse width means that extremely high power can be achieved at the pulse peak, which is particularly important for applications such as nonlinear optics.
Schematic of the regenerative amplifier system, femtosecond Ti:sapphire laser oscillator, stretcher and compressor. [12]
2. Ytterbium-doped femtosecond (Yb:KGW)
Since the 1990s, titanium sapphire crystal has become the crystal of choice for generating ultra-short pulses and ultra-intense pulses using chirped pulse amplification technology. While these developments have led to commercial products, new laser crystals have also been investigated to directly reach other wavelength ranges and overcome the need to develop continuous or pulsed green lasers to pump titanium sapphire crystals. In order to be able to directly pump crystals with very efficient, high-power semiconductor lasers, new Yb3+-doped crystals were developed. Yb3+ doped crystals are special crystals that allow diode pumping [13,14]. Yb3+ has good laser performance. First, the ions' low quantum defects reduce thermal loading and thus thermal problems. Secondly, there are no additional parasitic energy levels in Yb3+, which avoids undesirable effects such as upconversion, excited state absorption, and concentration quenching. In addition, compared with neodymium-doped materials (also rare earth ion doped materials used in high-efficiency diode-pumped lasers), Yb-doped materials exhibit broader emission spectrum characteristics. This advantage makes them more attractive in the development of ultrafast lasers.
Taking our most commonly used Yb:KGW crystal as an example, its gain bandwidth is ~18 nm and can support sub-200fs pulses [15]. However, due to the narrowing of the gain of the amplifier, the pulse output from the Yb:KGW regenerative amplifier is usually compressed to 300-500 femtoseconds [16]. In order to obtain shorter pulses, several methods have been developed to expand the output spectrum (such as using dual Yb:KGW crystals with two different optical axes) [17,18]. However, this makes the system quite complex and large. Another strategy is nonlinear amplification, which broadens the spectrum through self-phase modulation (SPM) to alleviate the gain narrowing effect [19,20].
Yb:YAG crystal and Yb:KGW crystal
The structure of the Yb solid-state femtosecond laser amplifier is similar to that of the Ti gemstone. It is also mainly composed of an oscillator, a pulse stretcher, a regenerative amplifier and a grating compressor. The difference is that the absorption band in the Yb-doped medium (shown in the figure below, ranging from 900-980 nm) is covered by the high-power InGaAs laser diode. This enables the development of direct diode-pumped and efficient compact all-solid-state lasers.
Emission and absorption spectra of Yb:YAG and Yb:CaLGO [10]
The Yb:KGW femtosecond regenerative amplification system combines the excellent laser performance of yttrium aluminum garnet-doped yttrium (Yb:KGW) crystal with the efficient energy gain of regenerative amplification technology. Due to its wide bandwidth and high-efficiency laser conversion performance, it has become an ideal gain medium in femtosecond laser systems. This crystal not only supports the generation of extremely short femtosecond pulses, but also maintains beam quality and system stability during high-power laser amplification due to its good thermal properties.
In the Yb:KGW femtosecond regenerative amplification system, the pulse undergoes multiple cycles in the amplifier cavity with the help of the gain medium - Yb:KGW crystal. Each cycle significantly increases the energy of the pulse, achieving a substantial increase in the energy of a single pulse. The result of this amplification process is the generation of femtosecond pulses with extremely high peak powers, while maintaining extremely short pulse widths and excellent beam quality. This high-performance output makes the Yb:KGW femtosecond regenerative amplification system ideal for applications requiring fine control and high repetition rates.
Yb:KGW solid femtosecond laser amplifier schematic[15]
3. Comparing the parameters of the two, what are their respective advantages and disadvantages?
Regarding the choice of titanium sapphire and Yb laser, it is more determined from the perspective of cost and market mature products. The advantages of ytterbium-doped (Yb) materials are high efficiency and low heat load: Compared with titanium sapphire, ytterbium-doped gain media (such as Yb:YAG, Yb:KGW, etc.) usually have higher light-to-light conversion efficiency and lower heat load. This makes the Yb system more stable during high power operation and easier to cool. In addition, flexibility in pump sources: Ytterbium-doped materials can be effectively pumped by different types of pump sources (such as laser diodes), while titanium sapphire materials usually require more specialized pump sources (such as green or ultraviolet lasers). Since the excitation wavelength band is around 500 nm, it is difficult to directly excite it with commercially available laser diodes. Mainly the titanium:sapphire crystal power amplification process has extremely associated costs. Large size (diameter 100mm) doped titanium sapphire crystal has very high cost (60k EUR) and very long delivery time (>1 year), and in current titanium sapphire lasers, when the pump wavelength is 532 nm, the empirically estimated threshold is 10J/cm2. For safety reasons, laser manufacturers conservatively set the fluency to 1J/cm2, which is 10 times lower than the damage threshold (10J/cm2)111. This value corresponds to an extraction efficiency in the range of 30%, which is even lower than the saturation mobility of titanium sapphire and does not meet the optimal power amplification conditions. Such low pump fluency means low extraction efficiency during amplification and a huge waste of pump energy, which is the most expensive part of a Ti:sapphire amplifier. Furthermore, approximately 2x as many pump lasers are needed for a given output to extract energy. [twenty one]
This leads to the fact that if the Ti:Sapphire laser wants to obtain higher average power, it needs an ultra-high-power laser pump source, which will greatly increase the cost of the system. In addition, when the Ti:Sapphire laser operates at high repetition frequencies, thermal management is also a very big problem. The heat load in the gain medium increases significantly, and the thermal conductivity of Ti:Sapphire laser is relatively low, so effective thermal management and heat dissipation at high repetition frequencies becomes a problem. Excessive heat load will cause a thermal lens effect in the crystal, thereby affecting the quality and stability of the laser output, which can only be solved by replacing a large-sized titanium sapphire crystal, which again increases the cost of the system. But for high-power titanium sapphire lasers, in addition to price, there is another practical issue that must be considered-the "embargo."
Refer to the Commerce Control List (CCL) issued by the Bureau of Industry and Security (BIS) of the U.S. Department of Commerce. The CCL details the goods, software, and technologies that are subject to export control. Each item has a specific Export Control Classification Number (ECCN) that describes the item and indicates its licensing requirements. CCL is divided into ten categories, with each category further subdivided into five product groups. For lasers, the most relevant category is probably: 6. Sensors and Lasers, which covers a variety of sensors and laser devices, including high-power Ti:sapphire lasers.
Commerce Control List·CATEGORY 6 - SENSORS AND LASERS》The specific export restriction requirements for non-tunable pulse lasers in the 540-975nm band are as follows:
B.4. The output wavelength exceeds 540 nm but does not exceed 800 nm, and any of the following: b.4.a. "Pulse duration" is less than 1 ps and any of the following: b.4.a.1. Single pulse output energy exceeds 0.005J and "peak power" exceeds 5GW; or b.4.a.2. "Average output power" exceeds 20W;
B.5. The output wavelength exceeds 800 nm but does not exceed 975 nm, and any of the following: b.5.a. "Pulse duration" is less than 1 ps and any of the following: b.5.a.1. The single pulse output energy exceeds 0.005J and the "peak power" exceeds 5GW; or b.a.2. "Single transverse mode" output, "average output power" exceeds 20W; [22]
Therefore, for many applications that are not sensitive to pulse width, fiber or solid laser solutions using Yb under the same power parameters may be a more suitable choice. Yb systems perform better in high repetition frequency operation and have higher thermal conductivity and efficiency. As shown in the parameter example in the table above, for the 10-20W femtosecond lasers with the largest market demand, the HELIOS series of ytterbium-radium Yb lasers based on localized technology outperforms COHERENT's Ti sapphire laser ASTRELLA series in almost all parameters except single pulse energy and pulse width. The HELIOS series has higher average power, higher repetition frequency adjustment range, lower after-sales costs and smaller equipment size. If there is a demand for short pulses, MPC (HYPERION series) can also be used to compress the output pulse width of about 300fs to <50fs while ensuring a transmission efficiency of >90%. Another very important point is that even if there are high energy and high peak power requirements, in terms of price, using the HELIOS-HE series with HYPERION-HE can also meet the mJ level 40fs output, and even if the two are used together, the market price is more advantageous than the Ti Gem series with the same power.
Femtosecond Yb solid-state laser vs traditional Ti:sa femtosecond solid-state laser
Additionally, as laser technology evolves, researchers and engineers have been searching for, developing, and testing new materials and configurations of other types of gain media to overcome these limitations and provide additional performance options.
4. What is the market demand? Which application fields are they suitable for?
In addition to the common titanium sapphire (Ti:Sapphire) and ytterbium-doped (Yb) materials, there are also many crystal materials for certain special applications, such as erbium-doped (Er), molybdenum-doped (Mo), holmium-doped (Ho), thulium-doped (Tm), etc. Each material has its own unique advantages and limitations and is suitable for different application scenarios. When selecting a material crystal as the laser gain medium, key considerations include pulse width, output power, pump source availability and cost-effectiveness, and the needs of the specific application.
Ti:sapphire crystals are more popular in applications requiring extremely high temporal resolution or ultra-high peak power due to their ability to produce shorter pulses and wider tuning ranges.
The titanium sapphire (Ti:sapphire) femtosecond laser system is one of the most commonly used and effective methods in current laser technology to achieve peak power levels from TW (terawatts) to PW (petawatts). In these systems, the peak power is greatly increased by applying various pulse chirping and compression and multi-stage amplification techniques. Amplify initial low-power laser pulses to peak powers in the terawatt (TW) or even petawatt (PW) levels.
The seed light source (also called laser seed source) is the starting point of the entire laser amplification chain, and its performance directly affects the quality and stability of the final output laser. At this time, the characteristics of high single pulse energy and extremely short pulse width of titanium sapphire laser are very suitable for amplification to obtain higher peak power. Such systems rely on the wide bandwidth and high-power laser amplification capabilities of titanium sapphire crystals, combined with advanced laser pumping technology and optical design, to achieve extremely high peak powers. At this time, the pulse has extremely high energy density and extremely short duration, and is widely used in fields such as physics research, material processing, medical treatment, and military.
(A) Layout of the front end and compression section of the terawatt femtosecond Ti:Sapphire laser system. (B) Amplifier portion of the system.
The amplifier was pumped with two synchronized commercial Q-switched Nd:YAG lasers with a total energy level of 1.3 J to increase the output energy. With just two Ti:Sapphire amplifiers we achieved a net gain of 4x108. As a result, a laser pulse with a peak power of 1.5TW, an energy of 230mJ, and a pulse width of 150fs was produced. Since the laser can operate at a repetition rate of 10 Hz, it also produces an average power of 2W. [twenty three]
The Yb-doped system is more suitable in some applications that require high power/high repetition frequency but do not have such high energy requirements, such as ultrafast spectroscopy, dynamic process observation, and micro-nano processing of materials. For example, ultrafast spectroscopy is a technology that studies the dynamic processes of matter on extremely short time scales. It can reveal the rapid dynamic changes of molecules, atoms and even electrons. Many reaction processes in ultrafast spectroscopy applications occur at the ps/fs scale. In order to be able to analyze very fast processes, ultrafast spectroscopy requires extremely short pulse widths, also at the femtosecond (10−15 seconds) level. Such pulse widths can provide sufficient time resolution to observe and measure ultrafast processes such as molecular vibrations and electron transfer. This process usually does not have strict energy requirements, especially in the research of some biological samples and sensitive materials, where conventional μJ or even nJ levels can complete the excitation of materials or components. An appropriate pulse repetition rate helps optimize the signal acquisition rate and thermal effect management, and is critical to improving experimental efficiency and avoiding sample damage. This is very suitable for the characteristics of the Yb laser system. According to different experimental needs, the pulse repetition rate of the Yb laser light source can range from kHz (kilohertz) to MHz (megahertz). Compared with the conventional kHz-level repetition frequency of titanium sapphire, the acquisition time is greatly shortened, and the experimental efficiency is increased by more than 10 times.
Schematic experimental setup of fundamental (black, dashed line), probe (green) and pump (orange) beam paths for broadband pulsed vibration spectroscopy.
SAP = Sapphire plate ND = Variable neutral density filter CM = Chirped mirror WP = Wedge prism. [twenty four]
Yb laser systems stand out for their high efficiency, low thermal load and direct pumping capabilities with high-power InGaAs laser diodes. Its advantage lies not only in its ability to provide stable high-power output, but also in its compact structure, which makes Yb laser systems ideal for laboratory and industrial applications. Especially in applications that require long-term operation and high stability, the unique properties of Yb laser allow it to surpass traditional titanium sapphire laser and open up new application areas. In the future, with the continuous improvement of laser performance requirements, Yb lasers are expected to play a more important role in the field of ultrafast lasers, pushing scientific research and industrial applications to new heights. Therefore, choosing a Yb laser system is not only an investment in current technology, but also a grasp of future potential, marking our entry into a new era of ultrafast laser technology.
In the future, with the development of new materials and the advancement of laser technology, we look forward to discovering more new laser gain media. These new materials will further improve the performance of femtosecond laser systems while reducing laser costs. In addition, improvements to existing systems, such as by increasing pump efficiency, optimizing thermal management or developing new pulse compression techniques, will also enable femtosecond laser technology to play a role in a wider range of applications in the future. Therefore, continuous research and innovation are the key to promoting the development of ultrafast laser technology, overcoming the limitations of existing technology, and opening up a new realm of laser technology!
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[22]Commerce Control List·CATEGORY 6 - SENSORS AND LASERS》Supplement No. 1 to Part 774 Export Administration Regulations Bureau of Industry and Security December 8, 2023
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